Hi Enrique,
Please try this.
DATA: ls_req_item TYPE bapi_te_requisition_item,
ls_extensionin TYPE bapiparex,
lt_extensionin TYPE STANDARD TABLE OF bapiparex.
*
ls_req_item-preq_item = lv_item_no.
ls_req_item-zfield1 = custom_field1.
ls_req_item-zfield2 = custom_field2.
*
ls_extensionin-structure = 'BAPI_TE_REQUISITION_ITEM'.
ls_extensionin+30 = ls_req_item.
APPEND ls_extensionin to lt_extensionin.
You could also do as mentioned in SCN Wiki - http://wiki.scn.sap.com/wiki/display/ABAP/BAPI
Regards,
Sharmila