please modified this script as per your requirement to add identity column to you existing table.
BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_PERSON
(
ID int NOT NULL IDENTITY (1, 1),
NAME varchar(25) NULL
) ON [PRIMARY]
GO
ALTER TABLE dbo.Tmp_PERSON SET (LOCK_ESCALATION = TABLE)
GO
SET IDENTITY_INSERT dbo.Tmp_PERSON ON
GO
IF EXISTS(SELECT * FROM dbo.PERSON)
EXEC('INSERT INTO dbo.Tmp_PERSON (ID, NAME)
SELECT ID, NAME FROM dbo.PERSON WITH (HOLDLOCK TABLOCKX)')
GO
SET IDENTITY_INSERT dbo.Tmp_PERSON OFF
GO
DROP TABLE dbo.PERSON
GO
EXECUTE sp_rename N'dbo.Tmp_PERSON', N'PERSON', 'OBJECT'
GO
COMMIT
for more information please review this link
SQL Server Add Remove Identity[
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OR
Sql Server Add Remove Identity 2[
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