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:
- One author publishes a large article
- Other writers create short-format comments
- Comments and replies are threaded together for context.
bbPress (and phpBB) are built to handle a discussion format:
- One author creates a topic (usually short-format)
- Other writers contribute responses to that topic
- Aside from the separation of individual topics, there is no other contextual threading
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.