With phpunit+Selenium, this is trivial to do. You just write a PHP method and call it from different tests.
With TikiTests, the only place where you can do this easily, is for specifying the start state of the app. Here, you may be able to reuse the same profiles or DB dumbs across different tests. But that's all. You can't reuse assertions, and you can't reuse sequences of screens. Well, you could, but you would have to add a lot of new features to TikiTests to support that.