Right now it's not possible to add just one Step to a Test Set/Run. In situations where just part of a Test needs to be run (because a small change was made or part of the area might be affected by something else and should be regression tested), the tester has to mark many Steps as NA. It would be easier to choose just the Steps that are in scope for testing from an are that a Test covers.

Example: A Test covers a form page, and a code change updates one of the radio button options. That radio button and the one that cascades from it should be tested, but not the whole form.