Create a new Contacts folder (File | New | Folder) then switch the view
on your existing folder to "By Category". Select the items in the
category you want to export and COPY those items to your new Contacts
folder you just created.
Not sure which version of Outlook you're using but probably what you'll
have to do is create a custom Contacts folder (temporarily) set your
Contacts view to "By Category" then copy (don't move) the contacts in
that category to the custom Contacts folder. Then do your export and
export that folder to the CSV file.