Using SqlMembershipProvider UserId as Reference for additional tables

Hello everyone:
Instead of creating a custom Membership Provider. I decided to create an additional table in the same database. 
The additional table uses the UserId from the the SqlMembershipProvider table; aspnet_Users as a Primary Key and then Reference the UserId in the aspnet_Users as a Foreign Key.
I created a Method using the SqlCommand Object which inserts a First and Last name into the new table when a UserName and Password are created in the aspnet_Users table.
My Question is, is this safe? I am a little worried about how the new table gets the UserId from aspnet_Users table. Is it possible that the wrong ID from aspnet_Users can be given to the wrong user. This will cause a user ID not to work.
He ...