Adding a new primary key ID column to an existing table
You can add your GUID ID column with the following.

ALTER TABLE [yourSchema].[yourTable] add [ID] UNIQUEIDENTIFIER DEFAULT NEWID() NOT NULL

To do this for multiple tables you could use something like this:

SELECT 'ALTER TABLE '+QUOTENAME(s.name)+'.'+QUOTENAME(t.name)+' add [ID] UNIQUEIDENTIFIER DEFAULT NEWID() NOT NULL'

FROM sys.schemas s
JOIN sys.tables t ON t.schema_id = s.schema_id

WHERE (     (s.name = 'Schema1' AND t.name IN ('Schema1_TableList'))
            OR
            (s.name = 'Schema2' AND t.name IN ('Schema2_TableList'))
            ... -- Add more schemas and their tables as needed
        )


This will give you all the ALTER TABLE statements to create an ID UNIQUEIDENTIFER coulmn with the ID values pre populated.