Attempting to create ASP.Net Core Identity data in an existing table with existing user records

Ok, I got this to work, not entirely sure why it works, seems like the AspIdentity fields are populated before they are committed. I have only added the specific changes that were the crux of the question, the DI, setup of the contexts, etc are elsewhere in the project.

Essentially, for each 'domain' record, I create a new ApplicationUser (IdentityUser-derived) record, and add the values for the custom fields in the domain record into the ApplicationUser record. Once this object is created, it has the necessary Asp.Net Identity fields already populated. I then get those fields and populate them back onto the domain specific record, then I save this. Once that is done, I re-find the record (this may not be necessary), and add the password based on the plain text password already in the domain record.

        var clients = new List<Client>(myService.myContext.Clients);

        foreach (var client in clients)
        {
            var user = new ApplicationUser // Derived from IdentityUser
            {
                UserID = client.UserID,
                FirstName = client.FirstName,
                LastName = client.LastName,
                UserName = client.UserName,
                PhoneNumber = client.Phone,
                Email = client.Email         // Other fields omitted for brevity.
            };

            var idUser = user as IdentityUser;
            client.AspIdentityId = user.Id;
            client.ConcurrencyStamp = user.ConcurrencyStamp;
            client.NormalizedEmail = user.NormalizedEmail;
            client.NormalizedUserName = user.NormalizedUserName;
            client.PasswordHash = user.PasswordHash;
            client.SecurityStamp = user.SecurityStamp;

            myService.myContext.SaveChanges();

            // Can we just use the 'user' object here?
            var newUser = await myService.UserManager.FindByIdAsync(client.AspIdentityId);
            var result = await myService.UserManager.AddPasswordAsync(newUser, client.Password);
       }

Hope this helps someone, this was important to me to get done this way.