This question is in regards to multisite SSL. I know I can

This question is in regards to multisite SSL.
I know I can get a wildcard SSL for subdomains, but if I have 100’s of standalone domain names that will not help. So my question is, is there a way to ‘force’ all checkout processes to go through the persons subdomain on our base URL instead of it all being on their domain name. Basically like how Shopify has it.
Here is the somewhat flow:
• Pro Sites, they sign up.
• They make it where it is their own domain name.
• They have the shopping enabled and therefore need SSL. So is there a way to make it by default that any domain name on the account holder automatically switches to subdomain on our domain for https?