Implementing another gateway not offered with Market Press plugin

Hi, I'm currently working on a site for client who has a merchant account with mercury payment systems. I would like to use Market Press since I've used it before and it worked great. The issue I'm having is that Market Press does not offer mercury payment systems as a payment option. Is there a way to easily implement another gateway into this plugin?

Any help would be awesome, even if you could point me to another plugin or other threads with the same question or issue.

Thank you very much in advance.