modifing Addon, without modifing source code !

Ha ! what a strange topic !
in fact i would like to modifie an Addon, by adding some fonction in several class …
but i would like not modifing directly the source code … just derivate classes so when addon is updated i do not have to rewrite all code…

so basicly i started derivating classes … but i am quite stuck … lack of knowledge.
when there is 1 class, it fine, but when several i am stuck … looks like i need to rewrite a lot … ( so my idear is useless, or not well implemented … )

I started to try modifie the “ofxSimpleGui” to add some controls … unsuccessfully

anyone can link me some kind of How To or tutorial , or exemple …
thanks