I'm assuming you are referring to something akin to what Microsoft does by assigning a KB identification number to the topic?
I don't believe there is anything presently available in RoboHelp for achieving this. If it's something you feel could be useful, please consider submitting a Feature Request to ask Adobe to add the feature. (Link is in my sig)
Helpful and Handy Links
I didn't think there was, but thought I'd better ask first! Strange how Adobe can market this as a tool for knowledge bases when you can't place reference numbers/tags automatically on each page (and have them update automatically!).
I have submitted a feature request, but if anyone else comes up with a whizzy way to do this, please please let me know!