Maybe you can use the OpenArgs optional parameter when you open the small
form to add the item not in the combo box list. Something like this:

NotInList Event:
Private Sub Combo0_NotInList(NewData As String, Response As Integer)
DoCmd.OpenForm "Form2", acNormal, , , acFormAdd, , Me.Combo0.Value
End Sub

Second Form Load Event:
Private Sub Form_Load()
Me.<<Your control here>>.Value = OpenArgs
End Sub

If you are getting the NotInList event to fire properly, this should work.