SD--Adding Custom Fields to VBAP and VBAK / Additional data B screen

choose the table VBAP or VBAK ( depending on header or item)

click on button "append Structures" .

it will propose you the standard name for the table ZAVBAP or ZAVBAK

add fields with prefix ZZ

trans SE51 maintain user exit screens in program SAPMV45A

8459 user exit item level

8309 user exit header level

you can then design your own layout. and define your flow logic

PBO modules MV45AOZZ

PAI modules MV45AIZZ

Data definitions MV45ATZZ

Subroutines in MV45AFZZ