Hi, good question….
There would need to be two steps to that: Converting from simplepress to a standalone phpBB, and then integrating that phpBB back to WP using WP-United.
A google search for a simplepress to phpBB converter turns up a few paid options (e.g. https://gconverter.com/simplepress-to-phpbb/), but I can’t vouch for their efficacy.
The best option is probably to convert via an intermediate step — e.g. this guy converts via bbPress: https://the-unsane.com/2012/converting-simplepress-to-phpbb3/
However what he says about passwords is a little incorrect, there is more to it than the simple substitution he suggests in his post, as phpBB converts characters like &, < and > in passwords. However, once you get WP-United installed after the conversion, it will handle these differences for you transparently and those users will be able to log in again.
If you go this route you’ll probably want to do some dry runs on a test site until you get the procedure down pat. It’s quite a project.
Hope this helps.
John