class Symbol def %(arg) ... end endThe above code does what you need, but leaves no hint that you've made a change to Symbol. An alternative solution is to define a module and include that module in Symbol.
module SymbolExtension def %(arg) ... end end Symbol.send :include, SymbolExtension
Granted, this isn't a huge hint, but if you check
Symbol.ancestorsyou'll find the following list.
Symbol SymbolExtension Object Kernel
extracted from: http://blog.jayfields.com/2007/01/class-reopening-hints.html