You can use deis domains to add or remove custom domains to your application:
$ deis domains:add hello.bacongobbler.com
Adding hello.bacongobbler.com to finest-woodshed... done
Once that's done, you can go into your DNS registrar and set up a CNAME from the new appname to the old one:
$ dig hello.deisapp.com
[...]
;; ANSWER SECTION:
hello.bacongobbler.com. 1759 IN CNAME finest-woodshed.deisapp.com.
finest-woodshed.deisapp.com. 270 IN A 172.17.8.100
!!! note Setting a CNAME for your root domain can cause issues. Setting your @ record to be a CNAME causes all traffic to go to the other domain, including mail and the SOA ("start-of-authority") records. It is highly recommended that you bind a subdomain to an application, however you can work around this by pointing the @ record to the address of the load balancer (if any).