Truth be told I'm also a little confused on how InstaMAT pulls the fonts as well, however I have found a way to import fonts if you know what you want to use!
In your package menu on the left side, select the "Open Resource" button, and select the font file you want to use.
Your font should come into InstaMAT as a package asset, like this.
You should be able to use that font now in any font-based editors in this project. 
Hopefully that'll satisfy your needs for the time being! I can see it becoming cumbersome to have to bring in many different fonts, perhaps there is a setting somewhere that you can path InstaMAT to your fonts folder but as of right now I'm unsure of that functionality. 🤔
Thanks, that worked! Also, I was able to open my font folder and just drag a new font in as well. In case anyone else is scratching their head on where to find their font (after it's loaded on the left side of the screen) you have to pick the "Show Objects from the Current Package" when looking for your font on the right side of the screen.
