Is there any other column or columns that can be used to uniquely identify
each row? If you need to update the new column with a number is there any
particular ordering or sequence? How do you plan to update this in the
future if it is not IDENTITY/GUID?

Assuming there is no meaning to the numbers and they will be just used as
unique identifiers, you can simply add the column as IDENTITY, which will
automatically populate the numbers for you:

CREATE TABLE Foo (
col2 CHAR(1));

INSERT INTO Foo VALUES('a');
INSERT INTO Foo VALUES('b');

GO

ALTER TABLE Foo ADD col1 INT IDENTITY(1, 1) NOT NULL;

GO

SELECT col1, col2
FROM Foo;

HTH,

Plamen Ratchev
http://www.SQLStudio.com

A table **by definition** must have a key.  What you have is a non-
sequential file written with SQL.  A kludge is to use IDENTITY or some
other non-relational numbering to make it into a sequential file; but
it is still not a table.

Since we have no idea what this "table" is supposed to be, we cannot
suggest a proper key for it.  In RDBMS, there is no such thing as
"magical, universal, one-size-fits-all" key; it has to be particular
to the table and the data element being modeled.