If you do not want to allow end users to link their own account then the admin can perform the link instead by following the steps below.
Disable self service linking
Select the primary connector from the Directories page and disable the self service account linking checkbox.
Link the Primary Account
Select the user's primary account to link too from the Identities page.
Select the secondary connector to link to and press the Link button
Find the Secondary Account
From the popup that appears enter the username to link.
The account is now linked and the end user can manage passwords for both accounts.