they havent put them on the credits only merle is confirmed that he will come and in talking dead they ask the director if they will show merle,morgan and duane and he said that there was 100% posability that 33% of that cast were gonna show up on the 3rd season
In the comics, Rick meets back up with them. Duane has been turned into a zombie and Morgan keeps him chained up because he doesn't want to kill his own son, or just leave him. Where the show is now though, it doesn't seem like they'll be meeting back up any time soon. Also, the show may (and should) deviate from the comics, so maybe Duane will live. It would be nice for Carl to have another kid to play with so he can stop wandering off all alone and messing everything up.
Rick gave them a police radio and said to contact him if ever in need. Eventually Rick will most likely be contacted by them. They are also heavily armed with many police firearms that were obtained when the two of them and Rick went to the station.