ReAssign InBasket work from one user to another

The easiest way to do this is to add another UserAction option; ie Approve, Reject, Reassign and if they select Reassign the flow would go to another UserAction node. If this is going to a Task level assignment then you just have to build the same category filtering rules for that Task. If you're going to a User level assignment then this becomes a little more complicated (but not impossible).

Possible solution:
You could add a textbox and a button to your form and after the user enters the new userid and clicks the button it triggers a web call (using ProcessFlow would be an easy way to do this) which adds a new record to the WorkUnit Metrics table and deletes their record. The work will now be in the new approver's Inbasket and out of the first user's. They may have to refresh their screen to see the update on their end.