trade storage slips for key items

[Zirconian]

to free up some space you could make it so to where the storage slips become key items so you wont have to hold onto them in actuality.

[Nyarlko]

They can't. The devs have actually gone into a lot of detail during a Freshly Picked livestream at one point explaining the actual coding behind how the slips work. Basically each slip contains a binary data string, and each 1/0 represents a specific item being stored on the slip or not. Since it's merely a true/false state, it's also the reason why items with variable augments are unable to be stored on slips. Reforged Relic armor has augments, but the augments only have a single state (the augments themselves don't ever change) and there are no versions of the items that don't have the augments, so those can be done. Anything that has augments that have value ranges can't be stored on a slip because there is no way to store the augment values.

Extrapolating from that, there'd be no way for slips to be converted to KI's. There's simply no way for a KI to hold the same number of bits that an item can. I'd even bet that KI's themselves are essentially nothing more than binary 1/0 flags, which wouldn't allow much room to hold the additional ~180 items that a slip can.