Hi
Dropdown for Rejection reason in Standard User Decision Task is not possible. You have two options:
1. Create a custom Webdynpro with APPROVE REJECT buttons and link it to approval task. Here you can populate the list box. Recommended
2. Easy one ! Dont worry about list box; provide outcomes in user decision. If you have only few (2-3) rejection reasons then you can try following approach
You can add multiple outcomes for Rejection with short rejection reason. You can explain each option in details in workitem text.
You will now see 5 options for approver. Not a best option from user experience point of view
Regards
Sandy