I just started playing with UI mods, but here's what i've found out so far.
Available fonts are listed in eq2ui_fonts.xml, you'll need to add entries to this one to add new fonts for use in the UI. I've only played with standard fonts, but to add a new one add something like this:
Code:
<TextStyle Algorithm="Simple" FontName="Times New Roman" Language="english" Name="Font_Euro_Zapf12" PointSize="12" UseCachedFont="true"/>
This would create a new 12pt font style called Font_Euro_Zapf12. You can also change Algorithm from simple to, for example, bold if you want bold text.
If you add the entry under the top Namespace element your new font will be available as (in this case) /Fonts.Font_Euro_Zapf12.
Hope this helps!