I don't think it is possible the way it works now, if you could convert the font to bitmaps, then you could probably write a script that packs it and writes the right code into the Json file. I think temmie actually used one when she made the game, because if you scroll down to the bottom of the Json file there is a link to a sprite packer or something. I can't quite remember though, so I'd recommend you check it out yourself.