We have a user who is already there in jive. As that user was a contractor, his id was created in jive with contractor username and email id. Now that user is no more contractor and he has the new user id with regular company.
While the option to migrate/merge two accounts is not available, depending on the specific configuration of your instance you might have the option to update the user's original account with a new username/email address. To confirm, I would recommend submitting a case in your secret customer group, here in the Jive Community so that the next available support engineer can review through your instance's configuration.