Dear Eugene, thank you very much for your answer, however, the Alt-Gr key and the Compose key are "different" ways to get those characters that are not primary (direct and shift) characters on a keyboard. My question referred to the former. I got an answer by reviewing the symbols file at /xkb/. Each group of special symbols has the following line at their ends, for example
partial alphanumeric _keys
xkb_symbols "intl" { // <--this is only an example
name[Group1]= "English (US, intl., with dead keys)";
// The key definitions go here
include "level3(ralt_switch)" // <-- I think this is the answer that I needed.
};
This article explains the differences between Alt-Gr (Classic way to access alternate characters in European KB Layouts) and Compose-key. For those that may need it, I am sure that you are already acquainted with it.