It is possible to set up a domain in another account as an external domain (DNS Transfer). Since the domain is already set up as an external domain (DNS Transfer) in one 1&1 account, the domain will have to be removed from this account before the option is available to perform a DNS Transfer of the domain in another account/package.
If you want to fully transfer a domain from another registrar to 1&1, you will have to complete aTransfer of Registrar.
This process does not cancel the registration for the domain as it is not registered with 1&1. This process will simply remove the domain from the account so that it can be added to another.