Having created two secondary accounts last night (and then having an internet outage) to complete my "test", I have been able to determine the following:
The only transfer function available is through Gmail, transferring contacts from one account to another. This existed before G+. There does not appear to be any kind of "merge" function, or "transfer circles" function associated with G+.
Therefore, I conclude that the only way this can be done is with the help of a Google representative. My assumption is that it will be an easier process if you do NOT have the destination email set up yet (your account name would simply be changed), but I am not Google and therefore cannot tell you definitively.
EDIT: +Ross Bagley
Is there any input you can provide on top of my experiment?