It sounds like you understand moving fonts from one deck to another, but you're looking to bring a font in from outside Decker? As in like a regular Windows TTF or whatever font?
Fonts in Decker are kind of their own format, that's not really transferable/convertible from other formats. So unfortunately you can't really just import in from an existing font, you'd have to write some sort of conversion tool. And I think a lot of modern fonts are vector-based which doesn't quite work with Decker's pixel thing.
I know Internet Janitor did do some stuff with converting over some old Windows 3.1 era fonts into Decker, and they're at https://beyondloom.com/decker/goofs/msfonts.html
If you don't mind pixelling in a font manually though then there's the fontedit.deck in the examples folder.
Let me know if I'm totally off-base, if you're just looking to move fonts from another deck let me know and I'll see if I can find some doco on that.