You can use fontbm to convert a ttf font to the required format, it's what I used to make the fonts for the game (I'm one of the authors). The font should be a pixel font and ideally a similar size to the one already there (although the engine will allow any size). There are some subtleties regarding the outlined version of the font (which I recommend having for legibility): when you create the font with fontbm you should add 1 pixel of padding in all directions, so that you can edit the resulting image to add a 1 pixel border. I made a script to do the outline for me, so I can help if needed.
One other thing to keep in mind is that the font should also support english text, otherwise the source will need to be modified to allow changing between fonts depending on the language chosen at the beginning.
