Therefore, I started to make it "real" not just a breadboard full of "dead bugs".
Looking into the code, I found a very interesting feature that provide 9 memories with only 3 buttons.
The solution is proposed by Dietmar, DL2SBA and consist in both hardware and software modifications of the original source code (which is already big and full of stuff).
Choosing to have Command Buttons AND DL2SBA bankswitch resulted into error.
I gave it a shot and when Command Buttons feature and DL2SBA bankswitch feature are selected, the compiling ends into
"exit status 1
invalid preprocessing directive #elseif"
somewhere around the line #8632...
Therefore, I commented the bankswitch and get to my workaround!
And this is my solution, below.
It's just a "shift" of the voltages read by the ADC; each position of the switch is "moving" the voltage with 3 memory places.
Therefore, we have the same 3 banks but with no software hassle.
Just check to see if in the "keyer_settings.h" there are at least 10 buttons (1 COMMAND and 9 memory).
Basically, my solution is similar to what Dieter proposed but it uses the original code without rewrite the variables on the ADC array.
And gave no compile errors, of course.