if the table has a primary key field that is not seen by the user, plus one
or more fields that show up in the form's combo box droplist, then just add
a record to the table with a primary key value and no data in the other
field(s).
alternately, you could probably use a Union query, for example
SELECT Table1.ID, Table1.someName
FROM Table1
UNION SELECT Null, ""
FROM Table1
ORDER BY Table1.someName;