Sure, here is how to add a column and modify its data in SQL:

SQL

ALTER TABLE table_name
ADD column_name data_type;

UPDATE table_name
SET column_name = new_value
WHERE condition;

For example, to add a column called Country to the Customers table and modify its data, you would use the following code:

SQL

ALTER TABLE Customers
ADD Country VARCHAR(255);

UPDATE Customers
SET Country = 'United States'
WHERE Country IS NULL;

This code will first add a column called Country to the Customers table. The Country column will be a VARCHAR with a maximum length of 255 characters.

The second part of the code will update the Country column for all rows where the Country column is null. The Country column will be set to United States for all rows where the Country column is null.

To run the code, you can copy and paste it into a SQL query tool, such as SQL Server Management Studio or MySQL Workbench.