Allow the possibility in a Test Plan to add an iteration that is not included in the Project Timeline

Context: In ETM, we would like to do some non regression test about a widget in ERM. These tests would be run on different environment (DEV, PROD,...) and each time there is a new version of the widget (V1.1, V2.1, V3,...). I am working in a project area for non regression text about the ALM. So I created a timeline to gather the tests of this specific widget with different iteration.

Problem: In the Test Plan, I added a test environment to specify the version of the ELM and the server (DEV or PROD). I was planning to use an iteration from my timeline but I am facing the following issue : in the Test Schedule section, we can only Browse an Iteration that is under the Development Timeline. Moreover, the Iteration of a Test Record is not reportable (from RPE or directly from the API). 

Impact: If I want to add the iteration of my Widget Timeline, I have to go to the Project Area parameter to change the Project Timeline, then add the iteration in the Test Plan and go back to change the Project Timeline. Moreover, a simple user who does not have the rights to change the Development Timeline cannot change it. 

Goal of the idea: It should be possible to select any iteration under any Timeline in the Test Schedule section of the Test Plan. An example would be in EWM, it is possible to select any Planned For in a work item no matter which Timeline is selected as the Project one.

Why it is useful ?: It allows any user to easily add the iteration of any Timeline in the Test Schedule section of a Test Plan.

How should it work ?: In the Test Schedule section of a test plan, when we click on "Browse", we see the list of Timelines and Iteration and we can select any Iteration.