Hi Luis,
You need to provide index to modify internal table.
Loop at IT_ALV into wa_ALV.
lv_index = sy-tabix.
Read table IT_ALV2 into WA_ALV2 with key J_3GBELNRI = WA_ALV-J_3GBELNRI J_3GPOSNRI = WA_ALV-J_3GPOSNRI
WA_ALV2-j_3gemppsp = WA_ALV-J_3GEMPPSP.
WA_ALV2-J_3GEMPKOS = WA_ALV-J_3GEMPKOS.
Modify IT_ALV2 from WA_ALV2 index lv_index transporting j_3gemppsp J_3GEMPKOS.
ENDLOOP.