Sorry: the connector does one-way-sync only.
You can try to connect an existing o365-user to a UCS-user by setting the UCS-users email address to the email address of the o365-user. It will then try to create or modify the o365-user, and hopefully create the required ID and thus the connection.
Attention: this is untested and it might fail at connecting the users or worse damage the existing o365-user! So test this with a fresh test user 1st created at o365 side.