Good news, everybody! It's possible.
Bad news, everybody! It could take a while.
It's only quick and easy for monospace fonts, as there's no easy way to import the character glyph widths so individual widths would need to be set manually which would take... a while.
Really the best way would be to write a converter to swizzle the Playdate FNT+PNG into BDF, which BitsNPicas can load directly maintaining all font details.
Did I see that there is a new font tool on the way that will save as BDF? If so, maybe this will get a lot easier fairly soon?
Anyway! Guide is here: Alternative Bitmap Font Tools - #2 by matt