During PR creation, system will take the PDT from material master, there are no option to set PDT for material group during PR creation.
We can maintain PDT for material group (with vendor combination) at info record level which will flow in PO/contract/SA etc but not in PR.
You may have to use an user exit or badi, where you will maintain the PDT with material group combination in a Z table and system will take the PDT as per material group form the Z table via the user exit or badi during PR creation. You can use the badi ME_PROCESS_REQ_CUST to write your own logic.