Ya font_add crashes, most likely cuz it never packs or renders glyphs. Canthelp was discussing this before with his python IDE rasterizing a font to hand to ENIGMA like the plugin does. ENIGMA needs to be able to render fonts itself.
font_add_sprite_ext(spr, string_map, prop, sep);
Also needs some thought because it takes a string that maps the characters to the glyphs.