Yes, you can certainly port your number from an existing AT&T landline to Comcast CDV as long as both are in the same rate center.
Normally ordering Comcast CDV would initiate the port request, which AT&T would release. It can take a few business days to weeks depending on any issues pop up like name mismatches. You would also be routed to a 3rd party clearing center to confirm and prove you did place the order and you can identify ownership of the old number.
Do not initiate any disconnection of the old phone number and old account until the port is complete. The old line will be disconnected a few days to a week after the port is complete.
Related question: if I establish CDV service with a new number, but later on decide to port over an existing number I have with another company, can that be done? How much does Comcast charge for that?
The FCC has changed its number porting rules to shorten the porting period for “simple” ports from the current four days to one business day. The new deadline applies to all simple ports, including “intermodal” ports such as wireline to wireless, wireless to wireline, wireline or wireless to VoIP or any other combination.
Filing a Complaint with the FCC
If you have a problem porting your phone number from one service provider to another, first try to resolve it with the responsible provider. If you cannot resolve the problem directly, you can file a complaint with the FCC. There is no charge for filing a complaint. You can file your complaint using an FCC online complaint form. You can also file your complaint with the FCC’s Consumer Center by calling 1-888-CALL-FCC You can also read the FCC Laws and file a complaint at the above link.