Which is more important from the user's perspective; the ID or the
description? Normally, IDs are internal to the system and are not displayed
to the user. Certainly, if all you're trying to do is change a relationship,
then the user need not see the ID at all. Here is some code that allows the
user to add a new description without having to concern themselves about
IDs. This code goes in the combo's NotInList event. You can get more
information about the NotInList event from here
http://www.pacificdb.com.au/MVP/Code/NIL.htm. Of course,this all assumes you
have a table in which all the Descriptions are stored.

Private Sub cboMyCombo_NotInList(NewData As String, Response As Integer)
   Dim strSQL As String

   DoCmd.Beep
   If vbYes = MsgBox "This appears to be a new Description." & vbCrLf & _
               "Do you want to add it to the list?", vbYesNo+vbQuestion, _
               "Add new Description"

       strSQL = "INSERT INTO myDescriptionList (Description) VALUES ('" &
NewData & "')"
       CurrentDb.Execute strSQL, dbFailOnError
       Response = acDataErrAdded
   Else
       Me!cboMyCombo.Undo
       Response = acDataErrContinue
   End If
End Sub

Regards,
Graham R Seach
Microsoft Access MVP
Sydney, Australia