I am very happy that you enjoy the lib :)
The font format is a custom one, and it is handled by pixelfont.h in the libs folder. The lib has a set of "builder" functions here, which are used to create a font in the format. Basically you call builder_create, then call builder_glyph for each character you want to add, and when you are done you call builder_font which gives you a pointer to a pixelfont structure. The first field in the structure is its size in bytes, and you can just save it as a binary blob, that can then be used in doslike by calling installuserfont. The same lib and builder is used in this other project, here, to build a pixelfont from ttf data using stb_truetype, but it is the same principle regardless what your input data is. What you probably want to do is make a standalone command line program that just includes pixelfont.h and makes your conversions.
let me know if it doesn't make sense, and feel free to ask followup questions!
and btw, I will clean up the readchar a bit at some point based on the issue logged in github, getting the full range should be easier, and besides, I probably want to look into unicode support as well, using UTF8.