If you wish to sell more than just a hosting account, you may choose to sell domains with your hosting plans.
This is done simply by navigating to Settings -> Products -> Products. If you have not already created a hosting package, you may do so by following these instructions. You will need to configure your domain services by following these instructions.
On the first page of the hosting package creation, you will see Include Additional Bundled Products. You click the drop-down, and select the product group with your domains. By default, this will be Domain Services.
Save your changes, complete the rest of the package configuration, and you have allowed domains to be bundled with the hosting packages.