possible in Odoo 18 with new option to choose a Warehouse from the website configurations or if empty, all warehouses qtys will be available online.

in addition:

• enable consignment (inventory settings) to be able to set owner on stored products
• create a internal warehouse i.e "WH/partnerX" to be able to make inventory adustments
• create a inventory adjustment with "partner" products, owner and qtys
• to update qtys its possible to export and import using "Inventory adjustment"
• • as the information from location and owner are in the ID of the inventory adjustment export, create a export (importable with ID) with counted quantity (the product and/or reference can be add to help), adjust the quantities and import ID and ‘Counted quantity’ and "set all"

dropshipment:

its also possible to add the WH/partnerX to "partner" vendor location (developer mode>vendor>sales & purchase tab) and configuring dropship route for the partner products, to delivery direct from "partner" to customers.

I hope it helps

Ricardo

Hello!

Did you find solution? Currently we faced same issues. Looking for customization.

I am currently working on a solution for this as well. My vendor supplies a qty in their warehouse which I would like to show as in my stock if I can drop ship it. 

I took a similar approach as to what you were thinking, I created a location in my warehouse called vendors, turned on the consignment inventory, and can now upload the vendors inventory in the locations report via an excel file. If on your excel file you use inventory_quantity_auto_apply and designate who the vendor is it will not be counted in your inventory valuation. Here is what I was able to import. 

My issue is for some reason 2 weeks ago I was able to update the inventory_quantity_auto_apply via an API but all of a sudden since then odoo is now not allowing that. This is what I had working two weeks ago anyone have any idea where I am going wrong?

Consignment stock is not your stock and is excluded from valuation. While you can sell the stock through ecommerce or Sales Orders

Maybe the documentation helps clarify

https://www.odoo.com/documentation/17.0/applications/inventory_and_mrp/inventory/management/misc/owned_stock.html