I'm in Moodle 1.9.11
If we were merging one Moodle into another the way we did it before was simply to leave the bigger Moodle as it was and then export using the backup tool within a course in Moodle including the user data and then restore that to the other Moodle.
I don't think this would work on the plp as it isn't course specific. The users may be in both Moodles before the migration. Their username is constant in both Moodles.
I can't see an internal Moodle tool that would back up the ILP data.
Is there a way of merging plp data from one Moodle to another? I'm keen to avoid tinkering about with the MySQL database as it isn't my favourite occupation and the chances for complete disaster are high.