Si tenemos una base de datos que ya existe pero deseamos trabajar con Identity como modelo de Autenticación, podemos emplear el siguiente script para la generación de las tablas:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetRoles](
    [Id] [nvarchar](128) NOT NULL,
    [Name] [nvarchar](256) NOT NULL,
CONSTRAINT [PK_dbo.AspNetRoles] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserClaims](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [UserId] [nvarchar](128) NOT NULL,
    [ClaimType] [nvarchar](max) NULL,
    [ClaimValue] [nvarchar](max) NULL,
CONSTRAINT [PK_dbo.AspNetUserClaims] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserLogins](
    [LoginProvider] [nvarchar](128) NOT NULL,
    [ProviderKey] [nvarchar](128) NOT NULL,
    [UserId] [nvarchar](128) NOT NULL,
CONSTRAINT [PK_dbo.AspNetUserLogins] PRIMARY KEY CLUSTERED
(
    [LoginProvider] ASC,
    [ProviderKey] ASC,
    [UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserRoles](
    [UserId] [nvarchar](128) NOT NULL,
    [RoleId] [nvarchar](128) NOT NULL,
CONSTRAINT [PK_dbo.AspNetUserRoles] PRIMARY KEY CLUSTERED
(
    [UserId] ASC,
    [RoleId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUsers](
    [Id] [nvarchar](128) NOT NULL,
    [Email] [nvarchar](256) NULL,
    [EmailConfirmed] [bit] NOT NULL,
    [PasswordHash] [nvarchar](max) NULL,
    [SecurityStamp] [nvarchar](max) NULL,
    [PhoneNumber] [nvarchar](max) NULL,
    [PhoneNumberConfirmed] [bit] NOT NULL,
    [TwoFactorEnabled] [bit] NOT NULL,
    [LockoutEndDateUtc] [datetime] NULL,
    [LockoutEnabled] [bit] NOT NULL,
    [AccessFailedCount] [int] NOT NULL,
    [UserName] [nvarchar](256) NOT NULL,
CONSTRAINT [PK_dbo.AspNetUsers] PRIMARY KEY CLUSTERED
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
ALTER TABLE [dbo].[AspNetUserClaims]  WITH CHECK ADD  CONSTRAINT [FK_dbo.AspNetUserClaims_dbo.AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserClaims] CHECK CONSTRAINT [FK_dbo.AspNetUserClaims_dbo.AspNetUsers_UserId]
GO
ALTER TABLE [dbo].[AspNetUserLogins]  WITH CHECK ADD  CONSTRAINT [FK_dbo.AspNetUserLogins_dbo.AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserLogins] CHECK CONSTRAINT [FK_dbo.AspNetUserLogins_dbo.AspNetUsers_UserId]
GO
ALTER TABLE [dbo].[AspNetUserRoles]  WITH CHECK ADD  CONSTRAINT [FK_dbo.AspNetUserRoles_dbo.AspNetRoles_RoleId] FOREIGN KEY([RoleId])
REFERENCES [dbo].[AspNetRoles] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserRoles] CHECK CONSTRAINT [FK_dbo.AspNetUserRoles_dbo.AspNetRoles_RoleId]
GO
ALTER TABLE [dbo].[AspNetUserRoles]  WITH CHECK ADD  CONSTRAINT [FK_dbo.AspNetUserRoles_dbo.AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserRoles] CHECK CONSTRAINT [FK_dbo.AspNetUserRoles_dbo.AspNetUsers_UserId]
GO
Felices Lineas