If you search the Access forums, you'll see some answers to this question. However, you can create a union query. This can only be done in SQL (create a new query and then select SQL View from the View button). Here's an example:
SELECT DISTINCT [County] FROM tblParcel WHERE [County] Is Not Null UNION Select '<All_Counties>' From tblParcel
ORDER BY [County];
Then place the query name on the RowSource of the UNbound combobox.
Then you have to test for the All condition in code. So on a command button's OnClick event or the combobox AfterUpdate event, you'd put something like:
If Me![countycombo].Value = "<All_Counties>" Then
WClause = "([County] Like " & "'*' Or [County] Is Null)"
Else
WClause = "[County] = " & "'" & Me![countycombo] & "'"
End If
The above is just an example.