HI Dihl
Program Exits are not meant to disrupt the Workflow Runtime LUW ....they are just meant to do additional work (preferably in Update Modules) based on the Lifecycle Events of a Work item. You can use it for above functions, however, it is not recommended.
For your requirement, Simply have the "Confirm End of Processing" checked in the Task definition of the decision. Once user selects a decision, the system will give a popup for confirmation before completing the WI. The standard has already provided this .. why trouble yourself with so much of complication (during development, testing and bug fixing) :-)
You would just have to copy the standard task TS00008267 and use your custom task with that check.
Another suggested approach (for fairly complex messages) is to:
1) Let the user Approve/Reject
2) Immediately after that, have a dialog work item step going to the actual agent of the above item - define the step (Activity/Form) with
"Advance With Dialog" ... this will show immediately after the user clicks approve/reject -> there you can give options to confirm. If not confirmed, loop back.
Regards,
Modak