Don't mess with an electric panel if you are not absolutely sure what you are doing. To add a circuit you need a breaker, wire sized to the new breaker and the outlets, lights or devices that are powered by the new circuit. If you have spare slots in the panel you need to get the proper breaker for the panel and just knock out the panel cover for the new breaker. If there is no space in the panel, you may be able to find a dual breaker that just takes up one space in the panel and substitute for an existing breaker.
You can swap a single breaker for two mini breakers or you can add a sub-panel. If you only need a couple of extra circuits then just add mini-breakers.
If you mean can you put a single 20 amp breaker in an electric panel, the answer is yes. An electric panel is typically made to handle more breakers with values that add up to more than the rating of the panel on the supposition that you will never draw full load on all breakers. If you do the main breaker would trip. In your case you are under utilizing the panel, but this is not a problem.
Contact a qualified electrician.
It is not in parallel. You put a breaker in existing panel and use that to feed the subpanel. The Amperage of this subfeed breaker should match the rating of the new panel. For example a 100A breaker might be typical. Remember that ground and neutral are only "bonded" at the main panel. Usually a subpanel has a means to separate the neutral and ground in a subpanel. Be careful since everything about doing this is dangerous.Another AnswerYes, you can have two breaker panels in parallel. If you had a 100 amp panel on a 200 amp service (or increase the size of the service, check with your power company on the size of your service, you could add a second 100 amp panel in parallel with the first. You can have up to 6 disconnects per service, but they must be located adjacent to each other or in the same enclosure. So either install the second panel beside the first or a 100 amp disconnect beside the first panel and feed out of the disconnect to the new panel located where you need it.
The amperage that can be used from a service distribution panel is governed by the amperage of the main breaker. If the main breaker is 70 amps then up to a maximum of 70 amps can be taken from the panel before the main breaker trips on an over current fault. Any combination of branch circuits can come off of this panel as long as there is slots left for breakers to be installed. If you add up all of the breaker amperages in your distribution panel now you will see that the total is well above what your main breaker is rated for. The main breaker is used for two purposes, one to disconnect the panel from the utility supply source and two to limit the amperage that cam be drawn from the manufactures rated equipment. If the electrical panel's maximum rated current draw is 70 amps then that is what the manufacturer of the panel found, through UL testing, that that is all it can handle without becoming overheated due to the excessive current draw.
Tandem breakers, often called split breakers or double breakers, provide two separate circuits in the space of a regular sized breaker opening. Every circuit breaker panel has a limited number of circuit openings available. The problem is that when the openings are all used up and you still need to add another circuit, what do you do? You could change the electrical panel or double up circuits on a breaker, but this could place too much load on a particular circuit. So what then? The answer that many have found is a tandem breaker. This type breaker is the same size as any other breaker, but it has its difference. This breaker sports two smaller breakers built into one regular sized breaker. Each has its own breaker switch and the breaker snaps in just like a regular breaker. With this simple innovation, you can add a circuit and protect the circuit on its own dedicated line.
First you need to determine if addition of the new breakers will exceed the 200A service under normal operating conditions. If so you will need to go to power company and increase service and put in a larger main panel. If you have the capacity for the extra current you can add a sub-panel and add the breakers there as well as the breakers you will remove from main panel to accommodate the sub-panel breaker. Another way depends on what breakers are in current panel. There are some breakers that can be duplexed in the same space as a single breaker.
New breakers can be purchased separately, and simply snapped into place in the modern breaker panels. Turn power off to panel before removing breaker panel plate. If you do not know what you're doing, save your life and hire an electrician to do it.
In the trade this is called a sub panel. It is a complicated job in as much as you have to know wire sizes, ampacity of wires and proper electrical workmanship. Such a project should be left to a licenced electrical contractor to take out the proper permits and call for proper inspections. By taking this route it will leave you confident that if any mishaps happen to the installation down the road your insurance company will be behind you 100%. In most places it is illegal for unlicenced person to alter electrical wiring
There are a few things to consider before doing this. If the branch circuits have a ground wire in the cable sets then they can be used again. Replacing a fuse box with a breaker panel is half the step. A fuse box suggests that the home is an older type home. The new breaker panel should not be traded straight across circuit for circuit but should be upgraded to at least a 200 amp panel these days. This will mean a new service mast, meter base and larger wire for the upgraded service. There will have to be an electrical permit taken out even for a fuse box to breaker panel change out. If there is a service disconnect switch ahead of the fuse box the job will be a bit simpler. Most good electrical contractors will give you a free estimate as to what your costs will be so you will be able to set a budget for the project. In most jurisdictions if you have work done on existing electrical installations the rest of the home has to be brought up to current code regulations. This will add to the expense depending on how old the home is.
The main breaker is at the top/center of the panel and should have a number on it. Usually 100 amp panels are used in homes because they are cheaper than 200 amp panels, and 100 amps is usually sufficient current to supply a home. If you plan to add any thing(s) that will draw much more current than an average home (machine shop, arc welder, etc.), you should look into upgrading the panel. Overloading a panel is asking for an electrical fire.