If it is just a matter of importing ONLY USERS from the other forum, then, You might need to play some SQL game. Couple of tables will do the trick. Make sure that you have email address uniqueness constrained there, and a way to deal with the exceptions.
Sorry to bump this one - I've recently been looking into acquiring another forum which is based on IPB 3.4 - can you confirm if I will be able to merge users and posts (and presumably forums) with my existing Xenforo installation? Are you aware of any problems with doing this on an existing installation - for instance the IPB has about 6 million posts, would I be ok leaving my XF forum open while these import? (presumably over about 24 hours)
Thanks for the quick reply - just one question though which doesn't seem covered - will the data import of a new board merge cleanly with existing data (no issues with duplicate accounts, or other ids), or does it delete/overwrite any existing data?