BAPI_PO_CHANGE add line

Hello

I have read all possible topics regarding my problem but still i did manage to add a line in my PO.

I want to copy the existing purchase order lines and add new lines with another purchase requisition.

I appended the po lines to the table POITEM and added de po item number in the POITEMX table but still i just get the error message : item XXX does not exist

It is refering to the item number that i gave for the items to be added.Which is the indicator telling the bapi to add the lines?

thank you