SQL Server 2008 :: Add Calculated Row Item In Same Column

I need to add a calculated column item in the same column. Please see SQL Codes for both existing data and desired outcome.

Product O is added according to:

for 201501 Product O= sum of en_count for product Y,W,N when yrmnth=201501
for 201502 Product O= sum of sum of en_count for product Y,W,N when yrmnth=20150

SQL:

--Existing Data
--===== If the test table already exists, drop it
IF OBJECT_ID('TempDB..#Table1') IS NOT NULL DROP TABLE #Table1

--===== Create the test table with
CREATE TABLE #Table1
(
product char(100),
yrmnth varchar(6),
en_count int,

[Code] ....

--Desired Outcome

IF OBJECT_ID('TempDB..#Table2') IS NOT NULL DROP TABLE #Table2

--===== Create the test table with
CREATE TABLE #Table2
(
product char(100),
yrmnth varchar(6),

[Code] ....