I can't imagine why you'd want the contacts in Excel, but here's one way:

Create another Contacts folder.  Switch the Contacts folder containing the
contacts you wish to export to the "By Category" view.  Select one category
header you wish to export.  CTRL-click the remaining category headers until
all the contacts you wish to export are selected.  Click Edit>Copy to
Folder.  Outlook will ask you for the destination folder.  Specify the new
Contacts folder you just created and click OK.  Outlook will tell you that
the action will apply to all the contacts in the categories you've selected.
Click OK.  When the copy finishes, export the new Contacts folder because it
will contain contacts only from those categories you copied.