I thought it did use such things -- there used to be a tool to handle the slight tweak on the format it used. Might have been another version though.
Otherwise yeah find the font and its graphical representation and alter that to be Cyrillic or decode the font handling enough that you can add new characters/valid decodings into the mix and add them in instead. The former is usually so much fiddling with a tile editor where the latter, which is usually the better outcome, needs to fiddle with the code and is not necessarily a project I would put upon those new to ROM hacking.