ok, well, assuming the full image is derivative of the low res image or vice versa
(i.e. Seaside.jpg > Seaside_full.jpg or Seaside-lowres.jpg > Seaside.jpg)
This is something that can be scripted
Then you'd loop through all the records for the nextgen gallery, inserting product records - presumably using the same category structure within woocommerce as you have established in nexgen. Your full-sized images would go in the woocommerce_uploads folder - this is protected by .htaccess to prevent unauthorized downloads, and the script would add the appropriate path for the full-sized image (while also importing the low-res images for the main thumbnail image). With the proper use of hooks, you could still use the nextgen gallery to display the images, but when you clicked on the thumb of the image, it would take you to the product page (or maybe you'd put a link on the single view page for nextgen)
Either way, it's not a simple process, but it's certainly doable.