You might be able to use the deffont tag command's face attribute for this, placing it an early init file prolly http://kirikirikag.sourceforge.net/contents/Tags.html#deffont (idk if this is different between different krkr versions)
I'm not sure how you can get the game to load custom fonts, if it's even easily possible, but you might be able to make your users install a font and then use its name ingame even without that. I do remember a game that definitely uses a custom font and is extractable: find the japanese trial for ことのはアムリラート, which I remember extracting the custom font from to use so my texthooker didn't spoil the plain text it was written in