Hi Paul!
If I understood correctly, you were looking for a way to automatically create the assign node during test setup, using an existing record in the database, through the View Data pane in Service Studio. Unfortunately, that is not possible.
Now, as you probably are already aware, we do promote the reusability of test code, which includes setup actions. This means that the test data created for a specific test should be encapsulated in its own OutSystems action, and that action can be reused by other tests in the future (you still need to create the assign first time, though). Also, if you need to setup a different record in the same entity, you can just copy-paste the action and change only the attributes you need in the new action.
If you haven't seen these yet, here's a link to a short video about Component Testing in OutSystems. It is part of the "Becoming a Tester in OutSystems" guided path
Hope this helps.