Is there a way to migrate a forum from fluxBB to PHP-Fusion?
Unless you can find a mod or script out there that already does this, or you can find a programmer to whip one up for you, you're going to have to get dirty.
Basically you'd want to export the current FluxBB database and then try and line up the tables and columns to their corresponding counterpart in the PHP-Fusion database schema.
Some cases might not match or have a counterpart, so your best judgement will be put to work here. After you tweak up the database, just import it into the new home.
It's not unpossible, but it may a little time.
Backup the FluxBB before you try and refit it to the destination though, just in case something goes horribly wrong.