Rich snippets MarketPress

I'm trying to find the structured data for rich snippets so I can fix it because it's giving errors and missing info.

This can be changed modifying the /wp-content/plugins/marketpress/includes/common/class-mp-product.php, but it's possible to do this without change the plugin's core files?

Also, I would like to add the "availability" and "itemcondition" properties?