Assuming I am correct and an account can be set up in this fashion I would try their support address which is: [email protected] . It wouldn't hurt contacting them directly in either case. I also believe you have to have a US or Canadian address and a credit card to use their "service".Last but not least if you do contact them ask them about a transfer fee-I believe they charge the person transfering out to another account. I could be wrong on these details however.
If all else fails you may have to get the seller to transfer it away from 1+1 to you.
p.s. their information hotline is 1-877-461-2631
Dongan: I just noticed the domain trade was cancelled so I guess you don't need this info afterall