OK, here's an idea for a cargo hold extension system.
Obviously it can't change a ship's hull shape, so would utilise some kind of future tech to compress the cargo.
The way it would work is by using nano-singularities to compress cargo canisters' gravitons into an abstracted state. A special interface would be needed in which we adjust the position of canisters in the hold to get three or more of the same type touching. This would create a high enough mass for the singularities to abstract the canisters' gravitons, thus causing them to crush down to a zero point state for transportation.
This system would be called Cargo Crush Singularity Actuated Graviton Abstraction, abbreviated to Cargo Crush S.A.G.A. Or something like that.
Cargo Offset Obligatory Kinetic Induction Energy System, abbreviated to C.O.O.K.I.E.S.
*Warning* This ship makes use of C.O.O.K.I.E.S., by loading cargo into it you accept the terms and conditions therein.