Yes you can edit the built-in font by editing the font_src.bit files in resources\shinonome and running generate_cxx_font.rb
Here is an old discussion where somebody added cyrillic to our font, gives some hints how to modify it: Cyrillic font problem
ExFont is not editable this way, it is a byte-array in exfont.h
If you add new glyphs that are not available in the Player yet feel free to submit a patch.
Complex scripts with combining diacritics (Thai, Arabian, …) are not supported yet, this will come together with the custom fonts.