You have not correctly answered his quest, imei repair for N960U is not supported with z3x box at the moment. That device is a qualcomm device, and hence would require professional service to replace imei. You need to ask your client what happened before imei got damaged, if it was naturally damaged by wrongly flashing an incompatible modem on device, then there are 100% chances of getting device original imei back, if you can root and follow certain procedures i won't be able to list here.
If you want more info on repair imei operation on N960U, write to me at Angle360.tech at the Help & Request section, i will be glad to answer your querry there.