I do not use this but I imagine you can add it if it is Bluetooth capable. Just go into the Samsung Gear Mgr app click on S Heath then select Setting, then click on my accessories and scan to add the BLE HR strap.
I'm not sure if the Gear S can connect to other BT devices beside BT headsets. I looked in Gear Mgr on the phone and there are a list of HR straps under my accessories to add. Since you do not want to use a phone have you tried looking under the Gear S settings, connection, then select bluetooth?