How to make A drop-down box editable by ALV function

HI GUYS !

     Today , I miss one question.  I  need provide AN drop-down BOX  in my program.   In default  THE drop-down BOX have three select.   Just like SLECT A

select B   and  c  .   BUT NOW  THE Customer require me add a  select which they can input what the want .  Now I  use the   function  'R EUSE_ALV_GRID_DISPLAY_LVC'

          But ...    I  dont know how to made it....

Anybady  Know it ? 

THANSK  FOR YOUR HELP!!