You seem to be dreaming of a seemless migration and I can assure you that you won't get it.
I don't know Steelbelt, but ACS 5.2 has a very very specific rule system and policies. So even if you migrate the user/device data, you still need to configure it.
To migrate user and device data, the simplest is CSV file import. If Steelbelt can export the users/devices in CSV, you can import them in ACS. The question is how can Steelbelt export its data ?
The configuration part of ACS, you will still have to do it manually. And I suggest not underestimating this because you need to understand how it works and it's not necessarily intuitive.
