This sounds very strange. You should not have an unswitched bathroom light. Replace fixture with one with a pull chain. Turn breaker off before changing the fixture.
If the question is, can I wire a switch to an existing light fixture (and I must assume that it's something like a pull-chain light fixture), the answer is yes.If the question is, "how do I wire a switch to an existing light fixture that has no external switch", the answer is, "in series".The attached web site gives simple instructions on how to wire a basic light switch to an existing light.See sources and related links below
Switching a light fixture requires the following steps. First, remove the old fixture. Second, you must disconnect the wiring. Third, a junction box must be installed. Finally, a new fixture must be installed. Detailed instructions can be found here: http://www.diynetwork.com/how-to/replace-a-bathroom-light-fixture/index.html
Two options. Change the fixture to a keyed pull-chain, or add a pull-chain adapter to the existing fixture. Both available from Home Depot and others for about $3.
Shut off the breaker that supplies the voltage to the fluorescent fixture. Wire the new pull chain switch in series with the black incoming wire and the black wire of the ballast. Turn the breaker back on and your switch will now control the off on operation of the light fixture.
To control a light with a switch, the switch has to be connected in series with the light fixture.
It just holds the bulb, does not have a switch in the fixture.
The light is wired the same as any voltage fixture. Bring the source voltage to the fixture and connect it to the two fixture wires. If you want to control the off-on of the fixture take the source voltage to a switch first and then out of the switch to the light fixture.
The device that holds a light bulb is called a keyed lampholder or a pull chain lampholder. These have the switch built into the base of the holder and have switch that looks like a key or a pull chain to operate the switch. Wire the continuous "hot' wire to the brass screw on the lamp holder (the other side of the switch is attached to the light socket at the factory). The neutral white wire will go onto the silver screw terminal. See related link below for some examples. There also pull-chain and other "canopy" switches that you could, in theory, install in an existing light fixture. However, it would almost certainly violate any UL or other safety listing on that fixture, perhaps causing problems with your inspector or insurance.
That's one I've never seen. Does the bathroom light turn ON when the bedroom light is switched Off? Does the bathroom light switch turn the bathroom light on and off normally when the bedroom light switch is left on? Need to answer these questions, then find where the bathroom light power comes from. It's power supply should either go directly from the power oanel to the bathroom light or to the bathroom light switch. The bathroom light switch merely acts as an interrupter for the hot leg of this power supply.
You have both a mechanical and electrical issue. From a mechanical standpoint you need the switch mounted securely in the fixture. The switch will have two black wires or two terminals. You need to insert the switch in series with the black wire going to the fixture. Let's say that the black wire from a supply is connected to the black wire in the fixture with a wirenut. Undo the two wires and connect each wire to a wire on the switch.