I have implemented it in my project, the main things you have to implement is UserStore and RoleStore
my SiteUser and SiteRole classes do not inherit from anything
the main thing is to add your own services before letting asp.net identity add its own services
services.TryAdd(ServiceDescriptor.Scoped<IUserStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserPasswordStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserEmailStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserLoginStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserRoleStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserClaimStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserPhoneNumberStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserLockoutStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IUserTwoFactorStore<SiteUser>, UserStore<SiteUser>>());
services.TryAdd(ServiceDescriptor.Scoped<IRoleStore<SiteRole>, RoleStore<SiteRole>>());
some of the same interfsaces will be registered here but it will use yours if they are registered first
services.AddIdentity<SiteUser, SiteRole>();