From what I see in their docs [0], you can either record samples directly from their hardware, or use a web tool [1] using Web MIDI (!) to update them. Guessing from the Web MIDI use, probably should be easy to create open tools or even load that page offline to update its samples.
Yes, you'd be able to use whatever samples you want; transfer from the computer ("drag and drop samples using the sample tool") or record them yourself via the input.
> MAKE USE OF THE CURATED SELECTION OF DRUMS, BASS AND KEYS THAT COME PRE-LOADED ON YOUR K.O.II.
Question: would I be stuck with their pre-loaded curated selection or is there any way I can "upload" any extra dum/bass/keys and use them?