Hi Mariahydro
I assume you have already setup your current site/warehouse/locations. Similar you would do for offsite storage locations. Create "Offsite" warehouse and add locations in the name of Supplier you are sending items to, to be kitted.
First you receive items to your current site from Origin supplier as you stated, you would then transfer stock via "Transfer journal" from current warehouse/location to one of your offsite warehouse/locations.
Once the items are kitted and ready to be sent back to you, you create additional transfer journal.
But like Crispin mentioned, if the items are kitted and coming back under different item number I would consider BOM lines and production order.
Kind Regards,
V