I don't know for sure, but I think modifying a plugin from a certain flatfile cms to work on another isn't easier than making a plugin based on an general open source shopping cart solution.
Or you can just integrate an external script, this is also more flexible if you would want to change cms at some point in the future.
This one is flatfile based: http://sourceforge.net/projects/quickcart/
Or you can just integrate an external script, this is also more flexible if you would want to change cms at some point in the future.
This one is flatfile based: http://sourceforge.net/projects/quickcart/