Two ways...

You could leave it as is but use a custom format. Right click on the
cell, choose Format Cells, pick Custom from the list and enter
###"."###"."#"."# in the Type box.

Or use a formula in another cell such as

=LEFT(A1,3)&"."&MID(A1,4,3)&"."&MID(A1,7,1)&"."&RI GHT(A1,1)

- John
www.JohnMichl.com