Yes, you can create extensions using the 3CX Call Control API. I assume you are referring the terms "users" and "extensions" to mean the same thing.
The provisioning link is a property you can also query from the API, but understand that in 15.5 the provisioning "files" are generated on-the-fly and "streamed" to the phones. Therefore, you cannot access the provisioning data via the API because the data doesn't exist except briefly when the phone requests it's provisioning information, and even then only in memory.