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To add a subquery to the WHERE clause in DB2 SQL, you would follow these steps:

1. Start the SELECT statement with the parent query, including the columns you want to retrieve.

2. Add the WHERE clause, specifying the conditions that you want to filter the data on.

3. Within the WHERE clause, add the subquery by enclosing it in parentheses.

4. Write the subquery as a separate SELECT statement, inside the parentheses.

5. In the subquery, specify the conditions that will return the data that you want to use in the main query.

6. End the subquery with a closing parenthesis.

7. Run the query.

Example:

SELECT * FROM sales WHERE customerid IN (SELECT customerid FROM customers WHERE country = 'USA')

In this example, the main query retrieves all columns from the "sales" table. The WHERE clause includes a subquery that selects all customer IDs from the "customers" table where the country is USA. The main query filters the sales data based on the customer IDs returned by the subquery.