You can't do that I'm affraid.
I've tried.
In the end you have to create a seperate email account and then create your new site using the new email account.
The next annoying thing is you have t deactivate your new domain from your current site.
wait at least 3 months then register the domain with your new email account.
Below is the information given to me by Microsoft Office Live Support in Feb 2009.
"
I understand that you want to cancel the domain 123xyz.com from your Microsoft Office Live account. You also want to know if you can use this domain on another Microsoft Office Live account. I realize the importance of your concern and I will provide you with the relevant information.
I have checked in our records and found that you have cancelled the domain 123xyz.com from your Microsoft Office Live account on 01/27/2009. Please note that it will take 90 days from the date of cancellation for the domain to be removed from Microsoft Office Live servers. After this period, you can re-delegate this domain on another Microsoft Office Live account. For this, you do not need to cancel your entire Microsoft Office Live account.
"
Regards
Matt