Hi Gennaro,
Since the field is in BSEG table only, so you actually don't need any CMOD code or anything. You just need to add the field in your extract structure and it will be extracted.
Please follow below:
1. Go to RSA6 - Enhance your extract structure with Field BEWAR only (remember NOT with ZZBEWAR) -> Activate the enhanced structure
2. Go again to RSA6 - Change your datasource 0FI_GL_4 (to unhide the field) - Uncheck both check boxes (shown in yellow below)
a) Hide Field
b) Field only known at exit
And save.
You don't need to do anything further the value will be extracted in datasource.
Thanks
Amit