I have a blog on Blogger that I would like to turn into a bbPress forum. This is a blog that has essentially been used as a forum, so bbPress is a better solution. Unfortunately, by default bbPress only seems to be able to import from phpBB and bbPress. I imported Blogger into WordPress as posts thinking I might be able to convert the posts to topics, with comments being the replies, but I can't seem to do that either. Is there any way to do this?
No, there isn't. And unfortunately that's because you've been using Blogger to do something it was never intended to do.
I'm not saying this is a bad thing.
Blogger (and WordPress) are built to handle an article format:
bbPress (and phpBB) are built to handle a discussion format:
Each tool is meant to be used in a different environment. Unfortunately, that also means there are very few (you're the second I've ever heard of) use cases of migrating from one to the other.
TLDR - there is no automated tool. Your safest bet is to do a manual migration (which will be time-consuming, I know). Your second best bet would be to hire a dev to script the migration for you.