[Plugin: WordPress eCommerce – MarketPress] Adding new gateways

I’d really like to purchase the pro version of MarketPress, but it doesn’t support my payment gateway (Elavon VirtualMerchant). Is it possible to add this gateway? The developer guide for the gateway is here:

https://www.myvirtualmerchant.com/VirtualMerchant/download/developerGuide.pdf

http://wordpress.org/extend/plugins/wordpress-ecommerce/