Hi
Please, see SAP Note 496083 - FAQ: Customer enhancements (User Exits) in purchasing, last question.
In the method FIELDSELECTION_HEADER (or ITEM), write this coding
read table ch_fieldselection assigning <fs>
with table key metafield = mmmfd_cust_01.
.........
if l_changeable = 'X'.
<fs>-fieldstatus = '.'. " READY FOR INPUT
else.
<fs>-fieldstatus = '*'. " view
endif.
endif.
It is related with ME_GUI_PO_CUST, method MAP_DYNPRO_FIELDS, when you add your field
field-symbols: <mapping> like line of ch_mapping.
loop at ch_mapping assigning <mapping>.
case <mapping>-fieldname.
when 'ZZFIELD'.
<mapping>-metafield = mmmfd_cust_01.
endcase.
endloop.
I hope this helps you
Regards
Eduardo