You can do that. One quirk of the system is that, if you decide to add a new workflow stage, it overwrites the existing workflow stages, so you’ll need to re-enter them in addition to adding the new stage(s).
Go to Admin > Maintenance > Data Maintenance and go to WorkflowStage at the bottom of the list.
Enter the new workflow stage you want to add, type in a description, and under Object Type, enter RMA.
Re-enter the workflow stages that already existed: Approved, Deleted, Pending Approval, Review.
And of course, hit that orange Submit button at the bottom.
Let us know if that doesn’t resolve it for you.
Have a great day, Regis,
Cetec ERP Support