Adding extra total column that sums all column values on each row
I'm not sure you need the CTE you're attempting.  Seems like you're doing all the work needed in your first query and can use THAT as the CTE.

Here's a mockup you may be able to just run:

I named your first column "Category" so that you'd have something to use in the final SELECT statement.  Also surrounded the values to be added in individual ISNULL([Value],0) so that any NULL values wouldn't cause trouble.

Hope this helps a bit.

WITH CTE_Data
AS
(
SELECT 'Total_Invoices' AS Category, SUM(T0.[DocTotal]) - (SUM(T0.[VatSum])-SUM(T0.[WTSum])) as "Amnt(Exl)", SUM(T0.[VatSum]) as "VAT", SUM(T0.[DocTotal]) as "Amnt(Incl)" FROM OINV T0 WHERE T0.[Series] &lt;&gt;'80'  AND
DateDiff(MM,T0.DocDate,'2017-02-02')=0
UNION ALL
SELECT 'Total_DownPayment', SUM(T0.[DocTotal]) - (SUM(T0.[VatSum])-SUM(T0.[WTSum])) as "Amnt(Exl)", SUM(T0.[VatSum]) as "VAT", SUM(T0.[DocTotal]) as "Amnt(Incl)" FROM ODPI T0 WHERE
DateDiff(MM,T0.DocDate,'2017-02-02')=0
UNION ALL
SELECT 'Total_CreditNotes', (SUM(T0.[VatSum])-SUM(T0.[WTSum]))-SUM(T0.[DocTotal]) as "Amnt(Exl)", -SUM(T0.[VatSum]) as "VAT", -SUM(T0.[DocTotal]) as "Amnt(Incl)" FROM ORIN T0 WHERE T0.[Series]='5' AND 
DateDiff(MM,T0.DocDate,'2017-02-02')=0
)

SELECT Category, 
       [Amnt(Exl)],
       [VAT],
       [Amnt(Incl)],
       ISNULL([Amnt(Exl)],0) + ISNULL([VAT],0) + ISNULL([Amnt(Incl)],0) AS SumRequested
FROM CTE_Data;