@BogdanH, what @pjd said is exactly what I had in mind.
First, you would create and save a "user voice" on your keyboard using its "voice set" functions, or whatever they're called. None of my entry-level Yamahas have that functionality, so I'm unfamiliar with it myself, except indirectly by reading what other people have written about it. My understanding is that when you select one of the preset voices, modify its parameters-- even as simple as just changing its volume or panning, but potentially including things like modifying the attack time, release time, filter cutoff, filter resonance, reverb depth, chorus depth, etc.-- and then save the custom settings to a "user voice," the keyboard creates a data file that has a particular extension such as "VCE," "LIV," and so on, as @pjd said. It doesn't contain any new sound samples the way a new voice from an expansion pack does; it's really just a MIDI file, except it doesn't contain any Note On/Off events, just the Bank Select, Program Change, and Control Change events which select a specific preset voice and then modify its voice parameters from their preset default values.
Second, you would save or copy that "user voice" to a USB thumb drive, copy or rename it to a file that has the standard "MID" extension, load it into a MIDI editor such as a DAW or MixMaster, change the values of the Bank Select and Program Change events, save the changes to a new "MID" file-- probably giving it a new name to reflect the "hidden" voice you're selecting with it-- and then copy or rename the "MID" file back to a "VCE" file on the USB thumb drive.
Finally, you would try selecting the modified "user voice" from the USB thumb drive to see if it successfully selects the "hidden" voice as intended.