i want to customize the workflow activity jsp page

I created the attribute called customer name inside the WTDocument. the value of the attribute should be updated once the activity get completed. 

i wanted to add the dropdown list inside the workflow activity task page, It contains some customer names like A, B C, D based on the selection of the user from the dropdown list it should be persist into the customer name attribute.
please help me to solve this. if some have any code snippet please share.