You can register it someplace else and have it directed here.
But I STRONGLY discourage you from using Comcast's tool to build your web pages. Maybe use some third-party tool and have it served from here. But do not use Comcast's PWP tools. They are inflexible, simplistic, and unsupported.
Before you use your Comcast space, you need to enable web pages on your account. Look above at Customers and Manage My Account.
As Beth said, Comcast won't take over your Domain name like normal web host providers, but you can get close. When you buy your Domain name, you can forward it to your Comcast website and there are generally two options - both of which start with your Domain Name provider (for example, GoDaddy.com):
- Simple Redirect
- Retain Domain Name
On a simple redirect, whenever a request comes in for your Fancy Domain Name it will substitute your Comcast URL and pass the request on. This means that your visitors can use your Fancy Domain Name when they type in the URL for your website. If they look at the Browser Address Bar, they will then see the Comcast URL. If they bookmark a page on your site, it will save the Comcast URL. This method allows you to use your Fancy Domain Name in conversation, on business cards, or wherever you want to tell someone about your site.
The other method retains your Fancy Domain Name in the address bar so a visitor would never see the Comcast URL. There's a technical tradeoff that I find unacceptable though. It is implemented using Frames, so that your site is occupying a Frame in a dummy page written out by your Domain Name Provider. Where it impacts is if a visitor bookmarks a page on your site or even does a Refresh, they will be returned to your Home Page. That is, instead of bookmarking the page someone is on, it bookmarks the first page they entered on.