Hi Buddy
Why not load the new custom sounds into Memory B and make a note on your sheet music that that is where they are.
In fact once those sounds (by which I presume you mean rhythms) are loaded into custom memory they are a lot safer there than in Memory A or B where they can be easily replaced by something else inadvertently.
Really you need a KN7000 and save everything to SD memory cards with amazingly quick loading speeds of around a couple of seconds.