How to add custom rows to an SQL query in MS Access?

In MS Access, you will need to use a kludge. This might work:

SELECT TOP 1 NULL AS ID, NULL AS Item
FROM (SELECT TOP 1 *
      FROM ITEMS
      ORDER BY ID
     ) as i
UNION ALL
SELECT Items.ID AS ID, Items.Item as Item
FROM Items INNER JOIN
     People
     ON Items.PID = People.PID
WHERE People.Name = 'John';

The only purpose of the subquery is to get a table with one row. Remember that TOP 1 in MS Access can return more than one row if there are ties.

Use a Dual table

SELECT NULL AS ID, NULL AS Item
FROM dual
UNION ALL
SELECT i.ID AS ID, i.Item AS Item
FROM Items i
INNER JOIN People p ON i.PID = p.PID
WHERE p.Name = 'John';

You need to specify what table you want your data from. Try this:

SELECT NULL AS ID, NULL AS Item
FROM Items INNER JOIN People ON Items.PID=People.PID
UNION 
SELECT Items.ID AS ID, Items.Item as Item
FROM Items INNER JOIN People ON Items.PID=People.PID
WHERE People.Name='John';

Perhaps if the union query order is changed to start with the formed query and the null query to follow:

SELECT Items.ID AS ID, Items.Item as Item
FROM Items 
INNER JOIN People 
ON Items.PID=People.PID
WHERE People.Name='John'

union 

SELECT NULL AS ID, NULL AS Item