you don't need to sign up for wearethird.mycafecommerce.com, if you contact Dreamhost support they will be able to add cafe commerce to your existing domain.
Any SSL Certificate that is implemented will be installed by the host. Dreamhost should be able to use a third party SSL certificate on your domain, but that is really a question for them to answer.
If you Open a support ticket with Dreamhost, they will be able to help you with setting up cafe commerce on your domain and adding the SSL Certificate.