I have brought a cellular Apple Watch but my current provider doesn’t support esimms. Am I able to pair it to my phone and activate the cellular option when I change providers ?

According to Set up cellular on Apple Watch.

"You can also set up cellular later from the Apple Watch app:

1. On your iPhone, open the Apple Watch app.
2. Tap the My Watch tab, then tap Cellular.
3. Tap Set Up Cellular.
4. Follow the instructions for your carrier. You might need to contact your carrier for help."

Yes you are able to pair the watch,it will function as if it is the GPS only version of the watch,and you can activate the cellular plan at a later date or not at all it's up to you.

Set up and pair Apple Watch

And when you decide to activate the cellular plan,here is how you do it;

Set up cellular on Apple Watch