Do you know if they're using an 802.16e WiMAX-based system? If they are, it MIGHT be possible. More recent firmware revisions for PMP450 allow for 5ms frame times, which make it possible to sync with PMP320 (which is 802.16e WiMAX-based).
There's a webinar here with some information on migrating from PMP320 to PMP450.
I've also attached a technical white paper from Cabmium on migration as well.
Good luck!