The GE add on switches work by being physically wired to a GE master switch. They have no network capability themselves. So they can’t work as you described.
However, there are auxiliary switches from other brands which are themselves Z wave devices and which can communicate with the SmartThings hub on their own. This can then create a virtual three-way so that the switch talks to the hub and The hub talks to the device that does actually control the load to the fixture. So good idea, you just need a different model device.
As it happens, the Linear auxiliaries are zwave devices and could be used for this.
In order to give the switches control of the fixture even if your SmartThings account is unavailable, you would need to do direct Association, which means you would also need a minimote to set up that association. The minimote can then also act as a handheld remote if you like. But it does mean buying one additional device. ( shop around for the minimote, you should be able to find one for about $20. )
If you don’t use direct Association, then if your SmartThings account is not available you won’t have any control of the fixture. So another alternative would be to get one master switch in the room that did control the load to the fixture so at least you would have that for control during hub outages.
( this all assumes that you do have a SmartThings hub to handle the message traffic. )