Adding Cellular to Apple Watch

No, you cannot. That is a hardware difference. The GPS only Watch will still work to handle phone calls, texts, and a number of other things as long as the paired iPhone is nearby. If you want to use the Watch with cellular when the paired iPhone is not nearby, you would have to purchase a new Watch. You need to make sure that you purchase the right model for your location and that your cellular provider supports the Apple Watch eSIM. See this listing, https://www.apple.com/watch/cellular/

No cellular radio chip in your Apple Watch.

If you want cellular, you will have to buy a cellular Apple Watch.