See my code examples for Presence sensor... device associated with local wifi - it’s doing essentially that by sensing device association with your local wifi AP as presence.
For sensors, you’ll want a single smartapp implementing an oauth REST endpoint, which has access to the “virtual sensors”.
You then write whatever scripts you want running on your local hardware which monitor the legacy sensors, and when they change values you use the REST endpoint to push the changed value to the virtual device inside of SmartThings. Everything else on the ST side will “just work” on value changes.