UPDATE: You need Connect.registerUsers:
This method is used to create an
association between an existing user
account on your site and that user's
Facebook account, provided the user
has not connected accounts before.
This method takes an array of account
data, including a required email_hash
and optional account data. For each
connected account, if the user exists,
the information is added to the set of
the user's connected accounts. If the
user has already authorized the site,
the connected account is added in the
confirmed state. If the user has not
yet authorized the site, the connected
account is added in the pending state.
If the user deactivates their external
user account, then call
connect.unregisterUsers. To get the
number of friends a user has in the
pending state, use
connect.getUnconnectedFriendsCount.
If that doesn't resolve your question, this question is not a duplicate of yours, but the answers might give you a way forward: