Why Would you want to do that instead of using SQL functions to do your SUM ?
You can very easily generate SQL to do the updates:
DECLARE @table nvarchar(100)
, @value INT
--- table name to update
SET @table = 'YourTable' -- set value to Your Table name
SET @value = 1
SELECT 'UPDATE ['+ TABLE_SCHEMA + '].['+ TABLE_NAME + '] SET [' + COLUMN_NAME + '] = '+ CAST(@value as nvarchar(3)) + ' WHERE [' + COLUMN_NAME + '] IS NULL OR [' + COLUMN_NAME + '] = NULL'
WHERE TABLE_NAME = @table
AND IS_NULLABLE = 'YES'
AND IS_NULLABLE = 'YES' AND DATA_TYPE NOT IN ('datetime2','datetime','smalldatetime','nvarchar')
Replace "YourTable" with the name of your table, and run. run the output into New Query window , you cal also modify the query to meet your needs .
SQL 2000/2005/2008/2012 DBA - MCTS/MCITP