We moved into a house that had all ADT items in the house. We went with another alarm system other than ADT. Can the ADT items like sensors, smoke detectors, etc be integrated with Smarthings?
Generally … No. - Different frequencies are used.
You may wish to contact SmartThings and/or ADT to see if there is some chance that the “SmartThings ADT Security Hub” (i.e., the hybrid ADT + SmartThings Hub) product is compatible with those sensors by some chance.