How to update VBAK-KVGR1 field value ? Overwrite TVV1 table by Ztable values.

Here in drop down of Customer group 1 field I want values from Ztable.

I checked domain of VBAK-KVGR1 field, TVV1 value table is maintained there.

I want to overwrite values of TVV1 table by Ztable values in VA02 Customer Group 1 field.

How can I do that? Please provide Exit or BADI if there.

Step by step solution will be appreciated.

Thank you in advance.