TrueType fonts can be added by coping them to:
/usr/share/fonts/truetype/
OpenType fonts need to be converted to TrueType by "fontforge":
fontforge -script otf2ttf.sh OpenTypeFont.otf
The converted font then needs to be copied to the above folder.
the script is:
#!/usr/local/bin/fontforge
# Quick and dirty hack: converts a font to truetype (.ttf)
Print("Opening "+$1);
Open($1);
Print("Saving "+$1:r+".ttf");
Generate($1:r+".ttf");
Quit(0);