Hi there! Thanks for checking in.
I just checked on this internally. Priorities can change, but right now it sounds like supporting adding your own custom icons is further down the road. It is a feature we are tracking, but it may not fit into the current projects. I would suggest exploring workarounds for the time being.
That said, we have gotten a lot of requests for icons recently (adding new icons to our icon list, search-ability, styling, etc) & we are exploring whether we can ship some lower lift icon improvements in the near term 
If I get any specifics, I will share them here