Mapping to the table is the easy part. Its the password encryption/decryption that will be difficult.
You might have to do a permissions migration over to claims or roles.
You simply create a user class with all the fields that match your current db table, and you'll have to add the new ones required by identity 2.0.
var mgr = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>());
public class ApplicationUser : IdentityUser
{
public string allMyNewFields {get; set;}
}
you will have to implement IPasswordHasher to get the passwords stuff.
Microsoft.AspNet.Identity.PasswordHasher : IPasswordHasher
checkout Asp.net Identity password hashing
good luck