You can add one custom domain to your Ghost(Pro) site. All requests for posts and pages on your site will then resolve to that domain.
If you have secondary domains (e.g.
yourotherdomain.com) that you would like to use with your publication, you will need to make provision for forwarding these requests to your Ghost site. This work takes place outside of Ghost(Pro) and you'll need to speak to your domain provider to find out the best way to achieve this.