Unfortunately, the only way to currently add keywords to an existing language is to modify the source yourself. To modify an existing language, or create your own, the best documentation currently is this thread
I am in the middle of a "developer's guide" that explains this material in more detail, with examples. It will describe how to add custom syntax highlighting, code folding, and code completion. I hope to have that material ready soon; I'll blog
about it and add it to the RSTA web site when it's available.