Thank you for contacting us.
Please elaborate a bit more on your scenario. What do you mean by "tests that were built by an external entity"?
When you add a test as step Test Studio finds the test by its ID. If you replace that test with another one it is expected that it cannot be found.
The workaround here is to add again the test as step.
If I am missing something please let me know.
Hope to hear from you soon.