You must call Rest-API POST in cycle. For example from csv line by line.
In powershell you can use Invoke-WebRequest function for each .csv line with Rest API call (body).
I have migrated all vShield5.5 to NSX 6.4.0 DFw. Manual parsing work over excel.
I have imported IPsets, Services, ServiceGroups .... by PowerNSX calls geenrated from excel.
For Firewall rules I have used Rest-API as I described above. The changing Etag after each call is the challenge.