"Depending on the age of the font" - now that we should all be making fonts the same way and giving glyphs true Unicode values, there is a "private use area" reserved specifically for stuff like this within Unicode. To write over the Cyrillic space is a no-no, but back in ye olden days people overwrote codepoints reserved for actual real languages (not LED bits) all the time. So I'm guessing that either your font is old, or poorly made. You should be able to use the Glyphs panel in either case.
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