Add default value all records new column

[Pedro Camara]

Hi guys
I have a query with a SELECT statement, joins 2 other queries, filter a column and order another one.
I would love to add a new column with a static value for all records, past and future.
I believe i should add these lines:

Alter Table dbo.CabecDoc
ADD Company VARCHAR (50) Not NULL
Constrain DF_Company default 'S&A'

But i don't know where, in the statement query....at the beginning, at the end, in the middle....

Can anyone help me please?
Thanks a lot all of you

P.S. If this isn't the right place to ask please let me know where