You donot need to Update RG1 register when you perform GRN against your Production / Process order as you will transfer the goods at a later point of time to some other Storage location (Say FG01 Finished goods storage location).
In this case, you have two options :
Option 1 :
Update RG1 against the transfer posting document in J1I5 transaction manually. Refer 315 Material document number, Movement type 315, Storage location (Say FG01) Posting Date, Classification RMA and execute the J1I5 report and Create entry in RG1 register.
Option 2 :
Update RG1 against the transfer posting document in J1I5 transaction manually. Refer 311 Material document number, Movement type 311, Storage location (Say FG01) Posting Date, Classification RMA and execute the J1I5 report and Create entry in RG1 register.
An of the above said options can be used for your scenario as per your requirement.