Simply trying to add the new domain, it reports:
Error! You can’t add that domain: already in our system
You need to delete the domain from your hosting (DNS) from the account first. Don’t worry, the domain name is still there in the other account. You just deleted the DNS hosting.
Steps to do this:
Login to the account that the domain is registered in, click “manage domains'” -> “‘delete”
to the right of the domain and confirm.
Then, you will be able to add the domain to the other dreamhost account.