adding new field to bapi segment

I have to edit ( to add new field ) in one bapi segement ( ZBP2080_*** ).

i had copied and tried it in we31 but it is giving a error message des. as below

Diagnosis

Segment type Z1BP2080_AW100 is flagged as generated.

System response

Generated segment types can only be displayed.

so plz can anyone give suggestion how to edit this segment .

thanks in advance .