Hey Dude,
I see some issue here. You have added these 2 new fields into VBAP. So users gonna maintain different values at item level in those fields? Normally STCEG is defined at customer level which means there will be one STCEG per order, hence we should add it to VBAK. But your requirement might be different.
OK. Let's assume that you intentionally added them to VBAP based on the requirement. Then in my opinion, you should be populating these 2 fields in KOMP structure and not in KOMK structure.
For that you can use userexit_kompbv1_fill in the Include RVCOMFZZ. ( Since you are populating VBAP fields, you can just pass COM_VBAP-STCEG to COM_PBV1-STCEG)
But if you still wanna populate KOMK, you can use userexit_komkbv1_fill in the same above include.
Cheers,
Tharindu.