I just signed up and that was an option. Don't know what to tell you.
And, btw, I just discovered in Invision that if you type @ followed by the first few characters of a member's handle it'll pop up the handles that match. Pick the appropriate one and s/he gets a message or email depending on how they have notifications set.
So yes, you can start with Nabble and then move somewhere. But you'll find that Nabble is so rarely used that very few know how to port it to their platform. But Invision now does as my 2300+ member site with an 8 GB backup file was moved yesterday and it came over with everything including our custom emoticons.