Yes, probably easiest to have a pull cord or something in the wardrobe (otherwise you'll probably be chasing another wire into the switch on the wall)
If you have loop-in wiring at the rose you can decide whether you want it to work indepandantly of the main light, or only when the main light is on, if you only have a switched feed to it, you have the former choice taken away (unless you find the loopin point and wire from there)
Anyway, take feed for the light to the new switch, then onto the light, both reds go to the switch, blacks together, and earths together and to any parts of the switch that require earthing (if any)