If you can't extend the mesh over to the building, then you can also run a second hub and use things like [RELEASE] HubConnect - Share Devices across Multiple Hubs (even SmartThings!) to bring devices from one hub back to your main one.
Hubitat also have a built in app for doing that as well, Link to Hub, which is a little less featured but works as well.
Edit: @jeubanks Shakes Fist 