Currently V2 supports full custom domains, but not subdomains of custom domains. If you don't want your store to serve as the homepage of your entire business, you can simply link to it from a "Shop Now" link (or similar wording) on your main business website.
You can also use a custom domain that's a variation on your main business domain. For example, let's say your business domain name is:
You could register one of the following domain names for your V2 store, and configure it as your custom domain:
Once your V2 store was up and running on that domain name, you could link to it from your main business website.