You can either use a formula in another cell, or translate them in place
automatically with an Event Macro:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim A As Range, t As Range
Dim v As String, s As String
Set A = Range("A:A")
Set t = Target
s = "-"
If Intersect(t, A) Is Nothing Then Exit Sub
v = t.Value
v = Left(v, 6) & s & Mid(v, 7, 2) & s & Mid(v, 9, 1) & s & Right(v, 4)
Application.EnableEvents = False
t.Value = v
Application.EnableEvents = True
End Sub
This will auto-translate entries made in column A.
Because it is worksheet code, it is very easy to install and use:
1. right-click the tab name near the bottom of the window
2. select View Code - this brings up a VBE window
3. paste the stuff in and close the VBE window
If you save the workbook, the macro will be saved with it.
To remove the macro:
1. bring up the VBE windows as above
2. clear the code out
3. close the VBE window
To learn more about macros in general, see:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
To learn more about Event Macros (worksheet code), see:
http://www.mvps.org/dmcritchie/excel/event.htm