Hello,
There is no setting for such customization of the URLs in JoomSEF. You would need to either modify the Content extension's code so that it creates URLs you need, or create some custom rewrite rules in .htaccess file.
We can help you with both, however you would need to purchase our support time for that. I think that the content extension modification would be easier and would take about 3 hours - if you're interested, you can buy our support time at
http://www.artio.net/en/eshop/support-services and contact us on
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Best regards,
ARTIO Support Team