Yes it is possible through Screen variant. Just follow the below process
Go to ME9F, then just press Save
Then enter the variant name and Meaning, then tick the option required field for purchasing group.
Then just press save. You can see the purchasing group field will be as required field.
If you want to assign it default, then go to SE93, enter the t-code ME9F, then press Change
Just give the variant name in "Start with variant"
Then press Save.
Now whenever you will run ME9F, your Purchasing group field will be as required field.