Saw in another of your postings you are using Moodle Workplace. The offer of provider (MWP) to do the merge is probably the only way to accomplish since MWP is restrictive about what plugins get installed and/or move foward/upward version wise.
Kinda curious why one would need a merge plugin of own for future merges, but then again, don't use MWP!
Sounds like (?) you are importing courses with users (in progress?) from other moodles?