Well, an existing Battle.net account can get recruited but they will have to create a new account under that Battle.net account in order for the recruit-a-friend to work.
For example, before recruit-a-friend:
bnetaccount[at]email.com
-TrialAccount
after recruit-a-friend:
bnetaccount[at]email.com
-TrialAccount
-RaFAccount <- this will be the one linked with the referrer.
"Our new referral system allows you to send friends and family members trial activation keys for World of Warcraft via email. With these keys, those whom you refer will be able to set up their own trial accounts completely free of charge. The trial period for these accounts lasts 10 days.
With this, trial accounts created through your referral will become "linked" with your account. This link will open up a number of benefits for characters on both the veteran and recruit account including increased experience, character summoning, and level gifting!"
Note that the trial accounts have no time restriction on them anymore.