Sorry, I thought you were talking about the stand-alone rte, not the rte in the forum.
For the rte in the forum, you need to either change and existing font or add a new one in two types of places - where the font is presented as an option, and where the option is expanded into code.
Files You'll need to change are...
forum_codes.asp
admin/functions/functions_format_post.asp
functions/functions_format_post.asp
functions/functions_edit_post.asp
includes/message_form_inc.asp
includes/RTE_toolbar_1.asp
That's for the rte in forum version 7.9