I think you're right. The modification of a test case can only happen at the test case level. And that sort of makes sense because the test script in the test player is in fact the test execution of a test case (they are separate things), and so if your test case is being executed multiple times by different testers in different test cycles, then a change to the test case should be made at the test case level so that it can be propagated to all test executions (if the 'update' option is clicked in the test player) - at least that how I think it works
In case there is a way to do this, you can try the smartbear support team at the link below. Please do share the solution if they give you one! 🙂
Support Portal