Spellcasting class with different stat for bonus spells

[Tashtego]

I'm entering a class from a third party product (the Elderkin from the Blackmoor player's guide.)

Their charisma determines the levels of the spells they can cast. (e.g. 16 charisma = level 6 spells). However, the class's bonus spells are based on wisdom, rather than charisma. I can't seem to find a way to make this work.

Any idea how to put this into Hero Lab?