Add quantity to GeneralJournalAccountEntry through posting vendor invoice process


I'm adding Purch/sales quantity field to GeneralJournalAccountEntry table through cust/vend invoicing process, in cust invoicing process it's working because all GJAEntries are created through class ledgerVoucherObject in method add trans, i found a class ledgerVoucherTransObject in wich i can parm Qty before adding to the list BUT in vend invoicing case it's working on some transactions but not all of them, Some of the transactions are created trough an other process like taxTrans and regularization.. i didn't succeed to debug this part of process.

Has anyone faced this problem ? please help 

Thanks in advance.