Calculate day gap and create a new column for the results

I would like to calculate the day gap and create a new column for the results. Here is my code: 
SELECT
DATEDIFF(day,FromDOS,ThruDOS) as LengthOfStay FROM tbl_Master

I want my new column to be added to the table I have queried. I have tried several coding options and scoured my SQL books to no avail. I know this must be simple. That is what makes it so frustrating.

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Here is some simple code for the table that has LengthofStay as a computed column to properly calculate the number of days between the two date columns: 
/****** Object:  Table [dbo].[tbl_Master]    Script Date: 
5/9/2005 9:45:17 AM ******/
CREATE TABLE [dbo].[tbl_Master] (
 [LOSID] [int] NOT NULL ,
 [FromDOS] [datetime] NOT NULL ,
 [ThruDOS] [datetime] NOT NULL ,
 [LengthofStay] AS (datediff(day,[FromDos],[ThruDos])) 
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[tbl_Master] WITH NOCHECK ADD 
 CONSTRAINT [PK_tbl_Master] PRIMARY KEY  CLUSTERED 
 (
  [LOSID]
 )  ON [PRIMARY] 
GO

Here is a screen shot with the sample data:

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This was first published in May 2005

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