Your ATPL skills test will be done on your sim course and you will be issued with an Omani ATPL on the strength of this. There will be no need to return to NZ unless you want a NZ ATPL. If you have a frozen ATPL on joining GF then there are no further exams required, you just go straight to your type rating. For those without ATPL exams a bridge course is done in Doha to cover this, it takes approx 6 months. The only thing you need to check is that the NZ licence is acceptable to the DGCAM, as far as I know it is so there should be no problem. This program is only available to Bahraini and Omani nationals.