subscription startdates

We have an existing mssql table of users from our real estate website that contain registration dates. We need to enter those old registration dates into the membership_relationship table so that the subscription expiration is based upon that date. As our users log into our website, I have managed to create wp users (using wp_insert_user) and pass our registration date to the wp_user table but I have not been able to do so for the relationship table. The current date is always used. Where in the code can I change this (wp users will always register thru our real estate website, not wp.)?