You need the proper tools and the qualifications to do this job. Call a qualified swimming pool electrician or pool tech to do the job to code and install a GFCI.
k
An electrical supply must be accessed and a wire fished to the fixture box. Do not attempt unless you are knowledgeable in electrical matters.
If you are adding the pull chain to an existing fixture then the pull chain switch should have two wires. Wire nut either one to the hot wire coming in and the other to the black wire of the fixture.
A nightlight is a light fixture placed in dark areas or areas that may become dark in an emergency. Electrical nightlights can be easily turned on and off at the flick of a switch.
A switched load is an electrical load that has a switch ahead of it. This allows the load to be isolated from the electrical source. It could be a safety switch used for isolating machinery when maintenance needs to be done. A lighting fixture in your house could be classed as a switched load.
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.
A tee joint in the electrical trade is a conduit fitting. It is used in conduit work where there needs to be a device connected into the main conduit run. An example of this is where a light switch needs to be installed into the conduit run. The main conduit run starts at the distribution panel and proceeds to a lighting fixture. On the way a switch is needed to operate the light fixture. In the main conduit run a tee fitting is installed so as to pick up the switch for the fixture. When the conduit system is complete the wire is then drawn in. The "hot" conductor is pulled to the tee fitting and then diverts to the switch box to pick up the switch and then back up the same conduit to the tee fitting and then on to the lighting fixture. Splices are not usually made in tee fittings.
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
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.
Installing a pull chain switch into an existing fluorescent lamp includes both mechanical and electrical issues. One has to change the wires and the switch mounts to have the two be compatible within the same fixture.
To control a light with a switch, the switch has to be connected in series with the light fixture.
For the most convenience, you should consider replacing two switches with just one switch that does both tasks. The first thing to do before repairing anything electrical is turn off the electricity to that room, then repair or replace the switch as you would any other light switch.