Adding a new primary key column

In SQL SERVER, I'm working with a table, I want to write something that will create an ID column if it doesn't exist and populate it with the next highest ID.
Query ::

IF NOT EXISTS 
(
    SELECT * 
    FROM   INFORMATION_SCHEMA.COLUMNS 
    WHERE  TABLE_NAME = 'SampleProduct'
    AND    COLUMN_NAME = 'SampleProductId'
) 
BEGIN 
    ALTER TABLE SampleProduct 
    ADD   SampleProductId   INT IDENTITY(1,1) NOT NULL 
END

This will assign identity values for all existing rows, and then provide new numbers as new rows are inserted.