As far as i know, there is no way to edit the .ffn file (open, edit and resave).
But a lot of EA games from that time used this font-format. So its possible to copy fonts over from other games, that also make use of this .ffn type.
Examples that work with NFS4 are fonts from: NFS3, NFS5, NBA99, FIFA97.
Not sure from which game the Muscle mod font originates, though.
The .ffn file is simply a texture map with characters (
similar to this Link). Guess it works like NFS3/4 HUD textures, where speedometer, rpm, symbols, etc are saved in one graphics file.
But again, to my knowledge there is no way to open/edit the file directly. Fontfiles from other games have to have same resolution and character order - otherwise you simply get rubbish, instead of text.
Greetings