First SSL's are overrated, they are not as secure as you think.
All you need to do here is put a SSL on the main domain yourdomain.com, and it will automatically transfer to operating on sub.yourdomain.com and any other domain.yourdomain.com as long as you have https://sub.yourdomain.com it will be secure.
Are you working with cpanel or some other control panel system. If cpanel this is a breeze.