Unfortunately, there is no way to add other group rate options in later. You'll need to delete and then recreate the event in DT Register. You don't need to delete it in JEvents. If you have a complex setup, a quicker solution might be to copy the event in JEvents. Then go to DT Register and copy the event there too... selecting your newly copied event from JEvents. Then you will already have your current details in place... then just add in the other group rate options. This is just like setting up a new event.
When finished, delete the original copies of the event in both DT Reg and JEvents. Thanks.