You can do via, but not for public transport, and indeed it magically hides the add destination button if you're not in one of the modes it supports (including if you're in the auto-mode and it's suggested a public transport route).

I can kind of see their reasoning, since public transport is often time dependent, but often I don't care about specifics, I just want to get an idea what links exist, and how long I'm going to spend in transport visiting a bunch of places.