Alternatively, if you want them to be changed permanently, then you can
enter this formula in B1, assuming your existing numbers start at A1:

=LEFT(A1,3)&"-"&MID(A1,4,2)&"-"&RIGHT(A1,4)

Copy the formula down for as many entries as you have in column A. You
can fix the values in column B by highlighting all the cells, then
<copy, then Edit | Paste Special | Values (check), OK and <Esc, and
then delete column A as you no longer need it.

Ron's method is a bit quicker!

Hope this helps.

Pete