Custom Registration to Modify JomSocial with verification from existing database

Hello Freelancers, I need an expert to modify a JomSocial registration system. One of the fields will be empty, and the registration (by email invite) must enter the correct credentials (compared/checked by system from an existing database imported into a separate table). If the credentials are correct, then he/she may proceed with the registration, otherwise it will give an error message and the person must contact the admin. The admin can update the existing previously imported database so that the person can resume registration with the correct credentials. Thank you.