(I hope I'm not too late) I use FireAlpaca/Medibang too, and the way you can do it is install it with the 'TrueType' instead of 'OpenType' (there are two types of the font you can install). Install the TrueType version of the font, then if you already have FireAlpaca open, you'll have to close it and open it again for it to register the installed font. Hope that helps!
Just tried it, and it didn't seem to work. Maybe I need to give it an hour or two to register. That's how it was with all of the other fonts that I've installed on my computer.
Hmm, maybe so. Well, either way, good luck! Hopefully they'll install!