Its DATEADD(MONTH, TestFrequency, TestDate) to add TestFrequency number of months to the date field TestDate .

SQL Server's TSQL equivalent to Oracle's PLSQL ADD_MONTHS function is DATEADD :

SELECT TestDate, 
       TestFrequency,
       DATEADD(mm, TestFrequency, TestDate)
  FROM TEST

I'm not exactly sure how Oracles Add_Months works, but MS Sql has this:

Declare @NumMonthsToAdd TinyInt Set @NumMonthsToAdd  = 6
   Declare @aDate DateTime Set @aDate = '12 Jan 2010'
   Select DateAdd(month, @numMonthstoAdd, @aDate)
      -- above will generate datetime of '12 July 2010'

CREATE FUNCTION [dbo].[ADD_MONTHS]
(
    @inDate SMALLDATETIME,
    @inFrequency INT

)
RETURNS DATETIME
AS
BEGIN
    RETURN DATEADD(MONTH, @inFrequency, @inDate)
END

-- TO Call : 
-- SELECT dbo.ADD_MONTHS(3,getdate()) AS newDate