"+ Create new slice" in prismic admin

Yeah sure you can edit the custom types in the dashboard as described here:

WARNING: This issue here is that if you start editing your Slicemachine Slice models in the dashboard this might create conflicts between Prismic and Slice Machine in your local project. Possibly stopping you from being able to push your updates to Prismic.

I'm guessing you want to do this to get the extra formatting options for the rich text you were discussing here. I would wait until these are pushed rather than editing in the dashboard.