Create firmware for Teensy 2.0 with multiplexer

[hexagown]

Hello, I have hand wired my custom keyboard. However, since it is an iso 105 layout, my teensy 2.0 does not have enough pins. Therefore I have brought a multiplexer, so I have 16 out of 26 rows and columns connected to the multiplexer.

Does anyone here have a guide or manual on how to rewrite tmk_keyboard matric.c-file (Or any suggestions on another firmware generator that is more suitable for this) to handle my multiplexing layout?