Hardware requirements for HomeKit compatibility?
Manufacturers can add HomeKit support through post-purchase updates but not many have as there are licensing and customer support costs associated with integrating with another smart home ecosystem.
You should look into HomeBridge. It’s a service that runs on a home server (iMac or Mac mini work great) and “bridges” the gap between non-HomeKit devices and HomeKit. For example, I have it setup and am able to control my Nest thermostat (temp and fan) from the Home app.