Do as mentioned below
sort itab by zprs_offshore.
Read table itab transporting no fields with key zprs_offshore = O.
Lv_tabix = sy-tabix
you will get index of fist offshore emplyee
Wa_itab-zprs_text = offshore
modify itab index lv_tabix from wa_itab transporting zprs_text.
similarly do for onsite in this way onlyrow for each onshore and offshore will get updated