Thank you very much for your post and for bringing this conversation to the forum.
Yes, the are called vendors and to be honest with you, I have bad and good news for you. The bad news is that we are not considering of developing any Vendor features ourselves as there is just too many and too good plugins already out there that can handle anything like this with ease.
WC Vendor, Dokan Vendor, Yith Multi-Vendor
The only think I can suggest is to chose one of those and in case you find an issue or an incompatible features, contact the developers and ask them to make their Vendor plugin compatible with ATUM.
Regarding custom fields - not as easy as you may thing and we would most certainly not create something just to patch up a feature. There is lot more about how inventory is handled in the WooCommerce code.
We have developed a Multi-Inventory premium add-on that has been released last month. There you can keep track of more inventories per product and each can have a different location. The only problem is giving access to some 3d party to it.
Hope this helped a little.