David - the menu itself is generated by whatever the configured menu provider is. So, in that respect, you can build your own menu provider, or take the existing one and extend it for your own needs.

A simpler, but less flexible option is to create a DNN page record for each product type. These pages would then use the 'external' Url option (at the bottom of the page settings). In here you would enter the path to the page which showed your specific product. When using this feature, the menu provider uses the external url supplied rather than what the 'page' url would normally be. In this way the menu entry points to the Url that you defined, rather than the one DNN would normally generate.
So you would have:
products -> which points to products.aspx
/products/producta -> which points to /products/productid/producta.aspx
/products/productb -> which points to /products/productid/productb.aspx

It's not flexible because you have to create each tab record to point to a specific Url, and adding new products won't include them in the menu. But for a small, static number of products this might be a useful solution.