Is your feature request related to a problem? Please describe.
I'm using Conveyance to move items around. The auto import feature of storage units grab items and puts these in the IN-slot. If the item isn't what the Storage Unit already contains, this locks up the Unit, making it unable to collect the item it's actually supposed to store.

Describe the solution you'd like
Storage Units should only import items which it contains (or any item if empty)

Describe alternatives you've considered
A block designed to route/filter items would be a nice alternative.