Fill "Segment" field when posting a document

I have to fill automatically the "Segment" field for each line when posting a document. For FB50/FB50L, I have added and enhancement in Include LFSKBF01, and it works, but, for FB01 transaction, I have not been able to find any badi/user exit that suits my requirements (I have already used the typical program that returns every user-exit/badi for a specified transaction).

I had thought in adding an enhancement in program SAPMF05A (the FB01 program), but, when I try to put some code in it, the system tells me "Statement not accessible.

Any help?

Thanks in advance 😉