If you really want to create an easy and user friendly workflow step, you cannot use the standard user decision step in your scenario. But you could for example include a "initial view" to your web dynpro component, and in this initial view you could have the options: Approve, Reject and Edit. From approve and reject buttons you would call for example the user decision completion function (search SAP_WAPI*DECISION*COMP* or just SAP_WAPI* functions) (this will complete the work item similarly to the standard user decision application), and from the Edit button you would just start your current editing application/functionality, etc.
After the above is done, you just change your UWL configuration in a way that instead of the standard user decision application, you will call your own web dynpro component, which then will take the responsibility for completing the work item.
Regards,
Karri