Found a work-around to transfer subscriptions to customer account and add domain admi
The only way I have found to correct the problem is to create a new customer with a fictitious domain name, then go back to the subs and transfer the subscription to the newly created account, then go back and delete the fake domain. You cannot set up the cust account with any email used in another user, so set up a gmail email to create each new customer, then after you do the transfer, delete the user with the gmail account before you can make another.
Time consuming and a waste of time but it is the only way I have found to separate the subscription accounts and give individual domain admin permissions to a single site. It works.
Also, be aware that after upgrading to version 10.x, that all spam settings are set to none, and any email accounts with forwards are lost, as are ALL auto-responders. Also, all of the filters with the auto-responders have been deleted and cannot be set back up again.
Do not attempt to create a customer in the business manager, as this will not allow the transfer of an existing subscription under any circumstances (tried them all). It will also try to lock out the domain, user and customer. I like the idea of the business manager, but I think it is only for new servers/accounts - do not try to migrate your old Plesk users as it will not work.
I'm assuming that Plesk and Igor have given up on this thread as there has been no response and they probably realize that they created a great big hole and fell in it. I guess version 11 will fix the issue. This is still a serious problem and it must be resolved. I have all of these domains that became subscriptions and there are all of these domain administrators who have access to multiple domains.