Setting the value of an execution variable within an execution record

I have created a project level execution variable. I have added that variable to a test suite and set its value. I would like to be able to adjust that value when the test suite execution record is created. I do not see a way of doing that. I do see that the value can be modified when the execution record is run.

Here is the scenario:

- Test developer creates test suite, adds project level execution variable and sets default value

- Test manager creates test suite execution record, and sets values for environment schedule and owner
                     Add this point, the test manager should be able to set the value of the execution variable

- Assigned tester runs the test suite execution record.  Tester should NOT change the values for the execution variable, which have been set by the test manager

Any idea on how to set the execution variable when the Test Suite Execution record is created?