Have a look at this:
Adding a font to ebook.py (Badger) - Support - Pimoroni Buccaneers
If want any special new characters just work out the bit pattern and substitute for one you do not use.
This might also help:
It is pure MicroPython and should be pretty easy to port over.
If you only want a few symbols you can just create them and place them in position. I will give it a go. How big do they need to be? 32x32 pixels should be OK?