Make sure the Combo's "limit to list" property is set to yes. Create a form to add the new data on. Then under "NotInList" event add something like this...
Private Sub cboCaseNumber_NotInList(NewData As String, Response As Integer)
On Error GoTo Err_cboCaseNumber_NotInList
Dim intNewCategory As Integer, strTitle As String, intMsgDialog As Integer
strTitle = "CASE NUMBER NOT FOUND"
intMsgDialog = vbYesNo + vbQuestion + vbDefaultButton1
intNewCategory = MsgBox("Do you want to add this Case Number?", intMsgDialog, strTitle)
If intNewCategory = vbYes Then
DoCmd.RunCommand acCmdUndo
DoCmd.OpenForm "formNewCaseNumber", acNormal, , , acFormAdd, acDialog, NewData
Response = acDataErrAdded
End If
Exit_cboCaseNumber_NotInList:
Exit Sub
Err_cboCaseNumber_NotInList:
MsgBox Err.Description
Resume Exit_cboCaseNumber_NotInList
End Sub
I just copied this from one of my databases...hope it helps.