Hi.
What you're trying to do is not supported in Isolator, with a good reason.
Tests should be plain about what thy are doing. When you try to "override" behaior, you make the test more complex, and even exposed to bugs.
What I suggest is that you collect all the common setup (that doesn't require special overriding) in a method, for exmple "Initialize". Then in every test, you'll add the special cases.
If you find that there are no common operations, just special ones, that means they weren't supposed to be together anyway. That means, if that method is [TestInitialize] or equivalent, remove the attribute and call it from the requiring tests.