Well, unfortunately we don't have a convertor that can do this in one step.
However, you could try to convert from IPS4 to MyBB 1.8, and then to phpBB 3.1. I have to say though, we can't provide any support for the first step, since we have no knowledge about the way that MyBB or its convertor (merging system) works. In theory, this way could work, but I can't provide assurances.
The convertor from IPS4 to MyBB 1.8 is listed here: https://www.mybb.com/download/merge-system/
The convertor from MyBB 1.8 to phpBB 3.1 is listed here: viewtopic.php?f=486&t=2278541