I suppose that not.. but is possible use the calendar feature from KMC WITHOUT a integration with Lotus or Exchange or OWA?
If you have a calendar without integration with Lotus/Exchange, what data would this calendar show? Do you have a proprietary groupware system? If yes, then you will have to implement your own groupware transports using the groupware API's. Use <a href="https://media.sdn.sap.com/html/submitted_docs/nw_kmc/howto/coll/How%20To%20Build%20a%20Groupware%20Connector.html">this</a> as a starting point for building your groupware connector.
If you have Lotus Domino or MS Exchange, then you can use the SAP shipped connectors to getthe groupware data in portals.