The best way to do this is to create a Custom Payment Gateway module.
See
http://www.excellencemagentoblog.com/magento-create-custom-payment-method
Magento Custom Payment Gateway
http://roysimkes.net/blog/2009/11/creating-your-own-payment-gateway-for-magento-part-1/
http://roysimkes.net/blog/2009/11/creating-your-own-payment-gateway-for-magento-%E2%80%93-part-2/