I'm trying to homebrew a weapon to change the damage die and use Wisdom for attack and damage rolls. Is there a way to do this?

You can do it directly on the character sheet. Check the attacks section, then click on Customize.

I was hoping for a way to build that into the weapon via a modifier, but I recognize that that probably isn't possible right now.

I ended up replacing the damage type of a longsword to get what I wanted, since my character is an elf and thus conveniently proficient in longswords.