…erm… you could very well do so, although you probably shouldn't. Key items are treated exactly the same as normal items (with the exception of no quantity count and the inability to toss/sell) and they DO have a quantity. You just have to deposit them one at a time.
If you are going to Item Underflow, don't put a key item in the 6th slot, or you won't be able to deposit all of it in the PC.
Yeah, but it's not really fun to deposit all 128 of my helix fossils in the pc.
You mean 129 (1 original + 128 duped)
Oh, right. :(