Wouldn’t it be easier to just do a report for Items where there is a supplier’s part number? Something like this? That way you just fill in the information on the Item page and you can quickly find out what a supplier provides, without having to have an extra doctype or a bunch of links.
If you really want to do it your way, you’ll need to add a bunch of python code to make sure those two lists are always synchronized. An add_fetch won’t work.