I will be starting a new extension soon that will enable you to import, export and update products in your Open Cart store using simple CSV files.
I have looked around the extension directory and notice that most of these fall short or only update limited information such as stock on hand, or base prices. My extension will attempt to work with most, if not all, of the core fields of a 1.5.6 +/- vanilla install.
Check back soon for more updates regarding this upcoming release.