I tried to see if you can save items by dropping them because it turns your dropped stuff into a backpack but it didn’t and I lost a bunch of ammo and a gun?
You can make another character (as a ‘mule’) and have them pick the items up to store them
The items will only persist within a session though (don’t quit right out), so you would drop the items from your main character (somewhere like a safe house) then > exit to main menu > select character > mule character > pick up items.
It’s a bit of a roundabout way to bank stuff though, and the GZ team have said they’re looking at item storage (but nothing confirmed as yet).