Hi Edgaras

Could you perhaps make use of the UserServices' SavingUser event?

I'm not sure what you would use for the password for the 'twinned' member?

public class AppStart : ApplicationEventHandler
{
    protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        UserService.SavingUser += UserService_SavingUser;
    }

    private void UserService_SavingUser(IUserService sender, Umbraco.Core.Events.SaveEventArgs<Umbraco.Core.Models.Membership.IUser> e)
    {
        var memberService = ApplicationContext.Current.Services.MemberService;
        // do something with each saved IUser
        foreach (var savedUser in e.SavedEntities)
        {
            //check if member exists
           var existingMember = memberService.GetByEmail(savedUser.Email);
            if (existingMember != null)
            {
                var newMember = memberService.CreateMemberWithIdentity(savedUser.Username, savedUser.Email, savedUser.Name, "Member");
                memberService.Save(newMember);      
            }
        }
    }
}