Yeah, that is frustrating. There is a loophole, though it would be better to not have to do it. If you get desperate enough the loophole is as follows:
Wyze offers a 30 day money back return policy, so you can do this at no extra cost: Order ANOTHER core starter kit with the HMS service AGAIN…then order all the extra sensors you want/need.
When your order arrives, contact customer service within 30 days and initiate the 30-day return policy on the second core starter kit and the second HMS subscription. But keep all your extra sensors.
You will get a refund for the starter kit and HMS subscription, and get to keep the extra sensors as if you were allowed to order them without a second starter kit. Problem [mostly] solved.
Disclaimer: I am not totally sure if they refund the shipping costs from the initial order, so you might lose out a little on that. I am not sure how they break that down or IF they refund shipping too. You’d have to ask someone who has done a 30-day return before if they refund shipping/taxes too. I’ve never personally done it, as I’ve always wanted to keep all the items I’ve ordered. 
If I were Wyze I would allow separate orders for individual sensors to HMS customers now so that they aren’t wasting a bunch of money in 30-day returns of starter kits pointlessly.