add light fixture to already existing fan light with switch at end of run

I don't know what R,B,W means. What country are you in? If it was an Australian installation, I'd be saying yes, but it's got nothing to do with the position if the run. I'd just be putting another lamp, in parallel to the existing one, in what ever position I wanted it. Would this adhere to the rules where you are? I don't know. One of the problems with asking trade questions, is that if you aren't a tradesman, then you're not using terminology that tradesmen have been using their whole working lives.

I think that's Red, Black, White.

In which case it's a wiring set up I'm not familiar with, such that the the OP should ignore what I wrote.

I recently did this myself. What I will write is in doubt contact an electrician.

Did someone miss the ground wire?

That's the part that throws me off.