With respect, you are barking up the wrong tree. Those utilities do not do what you think they do.
'bmtoa' does not create a font character. It takes an image file in a long-dead format and exports what is, in effect, an ascii art rendition of it, with '-' for 0 and '#' for 1. It has a very similar function to bmptoppm, but much, much more primitive. atobm does the inverse.
These mid-1980s tools do absolutely nothing that can't be done easier and better with Gimp (or MS Paint). They were obsolete as soon as xpm was devised in 1989.
'bitmap' and 'bmtoa' and 'atobm' are described fully in the O'Reilly X Window System bookshelf, Volume Three, "X Window System User's Guide", in the chapter entitled "Graphics Utilities" (which is either Chapter 6 or Chapter 7 or Chapter 8, depending on the edition). I have a copy here which I have consulted just now for the first time in ten or fifteen years. There are electronic copies out there, if you look, that O'Reilly presumably don't care about, since this stuff has been dead for so long. Note that some later editions don't cover bitmap/bmtoa/atobm, because they were obsolete and irrelevant; one such edition is on O'Reilly's FTP site, so don't bother with that one. There's another copy on archive.org...