Yes, you can transfer an external domain to easyname later on.
To transfer your domain, click on [Domains] → [Transfer domains] in your easyname Control Panel. On the following page enter the domain you want to transfer into the command line and start the query. Afterwards you will need to enter the "Auth-/Transfercode"of your domain and add it to your shopping cart. Simply complete the payment and the transfer of your domain will be initiated.
Please make sure that you do not enter your own name servers in the ordering process, as this will overwrite your already set domain settings.