I used to use 000domains.com, which is another dotster reseller, and I transfered all my 000domain.com domains to my mydomain.com account. You need 2 accounts through, one with your original dotster reseller (or big daddy themselves), and one with mydomain.com
It is easy to do. You have to first remove any domain transfer locks on the domains to move over, and then in the domain you want to move, there will be an option labelled something like "Move this domain to another account". Just move it your mydomain.com account name, and it will now be owned by your mydomain.com account and be removed from your old account.
It works great and doesn't cost anything, since you aren't actually transferring it.