I’ve been searching around but can’t find anything here apart from a couple of old topics without technical details of how it was achieved, if it ever was achieved. Here’s a list of
available convertors, they’re a bit ancient but may give you an idea or two such as converting Nodebb to something else first then converting to phpBB. The only solution I’ve found up to yet is a commercial one
here, the site is active so you may be in luck.
Edit: I see there’s a Nodebb to Xenforo convertor online, maybe you could convert to Xenforo first and then to phpBB?