Hi Dudu,
A little bit investigated, Since my PR is not configured with serial number! So I could not find the exact solution! However please debug the code below.
The serial number configuration is happening in include : LMEGUICJP.
in that two methods are available and check the enhancement spots, I guess it will fulfil your requirement.
METHOD serno_assign.
CHECK l_serno_manager IS BOUND AND
NOT my_item-sernp IS INITIAL.
l_header = l_item->get_requisition( ).
l_mereq_header = l_header->get_data( ).
my_item = l_item->get_data( ).
l_itemx = l_item->get_datax( ).
ENHANCEMENT-POINT EHP604_SERNO_ASSIGN_EBAN_01 SPOTS ES_SAPLMEGUI.
---------------
* call pop-up
ENHANCEMENT-SECTION EHP604_SERNO_ASSIGN_04 SPOTS ES_SAPLMEGUI.
CALL FUNCTION 'SERIAL_LISTE_PR'
ENDMETHOD.
METHOD serno_list.
ENDMETHOD.
Since i can't able to debug due to the test dat, I think you can able to find the soltuion with in the two methods.