Hello Jamie:
Just some thoughts:
1. Could you find a plugin to download the data from that site you want to download used products from in CSV format instead? This would allow you to follow Guust suggestion to use the build in bulk import tool.
2. Have you spoken to that plugins author? They have the best grasp on their codebase and it would be fastest to have them modify it. Potentially they could even offer a way to export to CSV ( which would be a value add it seems to their plugin) and being the plugin author it could benefit the community if it gets pushed into the core PLUS most likely would be the most cost effective way(they known their own code best) and being integrated into the core helps allow you to keep a maintainable solution and helps the community.
Which plugin are you trying to modify? Does the plugin keep constant updating / live sync / or is it a one time/manual plugin?