Hi
Yes - users elsewhere have reported that AT&T can add the AT&T Unlimited Wearable plan for Apple Watch Series 3 (GPS + Cellular) to their billing accounts, without altering or affecting their grandfathered plans.
The process involved (manually provisioning a new line on your billing account) is not the standard process, so may require referral to another AT&T representative who is more familiar with the required steps.
You will need to provide the EID and IMEI for your Apple Watch, both of which can be found on the bottom of the box that your Apple Watch came in. (There is no ICCID, as the embedded SIM / eSIM in Apple Watch copies information from the iPhone's SIM card).
An AT&T number that it is suggested elsewhere to call is 800 331 0500.