Regarding BSEG Posting

hi experts,

I have few custom fields in my BSEG table ,

like

yycns

yasd

yass

zzabd

Now i want to include these fields in my standard BAPI_ACC_DOCUMENT_POST

and den to enhance IDOC for this Bapi, Here IDOC is capturing data from ETC layer external to SAP.

How to process step by step. Can anybody send me example code for the same.

I will provide points for the help.Please reply fast as its a bit Urgent.

Regards

Amit Singla