Another way would be to implement the VENDOR_ADD_DATABAdI since its interface IF_EX_VENDOR_ADD_DATA explicitly offers the following methods:
- CHECK_ACCOUNT_NUMBER()
- MODIFY_ACCOUNT_NUMBER()
both coming with LIFNR as a parameter - which then could be used to fire the aforementioned SELECT - exporting its reslult to ABAP memory and retrieving it in the EXIT_SAPMF02K_001 user exit.
Chris