WI Template - The owner of the Workitem should be the approver - how to do this ?

If my understanding is correct you want to get the owner name as approvers name while you are giving for approving to  person.I think you need to use a script based condition in attribute customization thorugh rtc client or plugin for the same.

1. It does not make any sense to have the owner be an approver. If that is the business rule, at least my common sense would say: leave the approval out. It is meaningless anyway. I wonder where the requirement comes form. 
2. There is no way I am aware to do that without using the API.
3. It would be possible to tie ownership to a state change. Again, API. Example: https://rsjazz.wordpress.com/2014/05/26/only-owner-can-close-workitem-advisor/