You can't, at least not in a query. There has to be something there to
generate the row in the query.
However, if for example you wanted to have a list of all customers and their
total sales, whether or not they've bought something, you can use:
SELECT Customers.CustomerName, Nz(Sum(SalesAmount), 0) As TotalSales
FROM Customers LEFT JOIN Sales
ON Customers.CustomerID = Sales.CustomerID
Using a LEFT JOIN means that there will be one row for each entry in
Customers, whether or not there are matching rows in Sales.