Simple lua code help

[akgis]

Want to do the following

lets say we have a function called X in the middle of the code, I want to add code to that function X but adding it at the end of the Lua file or even better in another Lua file

In the practice what I want is to add all that code again to that function but instead of looking all over the addon and messing the code I just want to add my custom code and then just call a line or 2 in that function, that way I know my custom code its at the end of the file(or another file would be even better)