No I am sorry it is not possible to use a test from a different project with our Test-as-step feature. The technical problem is that Test Studio assumes any coded steps for that subtest will use the same DLL that the parent test is using. Since the test is contained in a different project it must use a different DLL, and Test Studio is incapable of locating that matching DLL.
As a result we simply do not allow this in our UI.
All the best,
the Telerik team