Thanks Prakash for the help , but i saw that E1EDKT2 also has a structure of its own. and E1EDKT2-tdline is what we need to alter and save back into the idoc_data table i reckon.
this is what i tried
loop at idoc_data
assigning <fs_idoc_data>
where SEGNAM = 'E1EDKT2'.
move <fs_idoc_data>-sdata to ls_e1edkt2 ( wa for e1edkt2).
concatenate 'xxxxxxxxx:' ls_e1edkt2-tdline into ls_e1edkt2-tdline separated by space.
move ls_e1edkt2-tdline to <fs_idoc_data>-sdata+6.
endloop.
do you think this will work?