Currently (AFAIK) it isn't posiible to highlight a word inside of a comment, but you can add your own keywords for stuff like "global_variable" "internal" etc.
I think that it's planned to add support for custom highlights in future versions though.
Everything nicoco said is right. You can add keywords but nothing in a comment is treated as a keyword. The plan is to get to a system sometime this year that will greatly open the customizability of how exactly the buffer is shown, which will include arbitrary coloring rules.