No exporting needed.  Connect to your Exchange mailbox from within Outlook,
make it the delivery location, then open the Personal Folders file where you
were keeping your Contacts, select them all with CTRL-A, right-click the
selection, drag it to the Contacts folder in the Exchange folders, release
the mouse button and choose Copy (or Move). Now your contacts will be where
they should be: on the Exchange server, visible both from within Outlook and
via Outlook Web Access.