You have a bunch of options. First would be to use a Z-Wave or Zigbee sensor that has external contacts that you can wire in the relay from a smoke or CO alarm. I did just that here: Integrating Kidde Smoke / CO Sensors into SmartThings Properly . This would be good if you already have a bunch of wired sensors and they have a relay option board you can add.
Second option is the same relay but plugged into a Konnected board. There is a little setup but the Konnected product is a nice “packaged” solution to get wired stuff into the hub.
Third option is the same relay but plugged into a ESP8266 or ESP32 using ST_Anything (more DIY then Konnected but cheaper and more options of what to bring in): [DEPRECATED] ST_Anything - Arduino/ESP8266/ESP32
Last option would be just putting in a additional Z-Wave smoke sensor and connecting it directly. Like this one from FirstAlert is a combo Smoke/CO for $36: https://www.amazon.com/First-Alert-Detector-Carbon-Monoxide/dp/B00KMHXFAI/ .
For monitoring you can use the “preferred” SmartThings Scout monitoring or NoonLight. Or switch to the ADT hub and sell your v2 hub if you can get everything at a decent price. I think yesterday the ADT hub was $100.
So cheapest/easiest I would think would be a extra Z-Wave smoke/CO, your existing sensors, and NoonLight but at least you have lots of options.