i'm using 10.1.1 and am unable to update to 10.2, so i needed a work-'round for the 'can't create a new customer account for a domain already in plesk under another customer' problem.
the work 'round was: i first created the customer account for a domain NOT in plesk (i suspect the domain doesn't even need to exist); i then transferred the existing domain to the new customer account; i then removed the first (non-existent) domain from the new customer account.
it seems to have worked out ok.