Have you looked at the vfiler migration cmdlets?
PS C:\> Get-NaHelp *VfilerMigrate
Name Api Category
---- --- --------
Complete-NaVfilerMigrate {vfiler-migrate-complete} vfiler
Get-NaVfilerMigrate {vfiler-migrate-status} vfiler
Start-NaVfilerMigrate {vfiler-migrate-start} vfiler
Stop-NaVfilerMigrate {vfiler-migrate-cancel} vfiler