Properly Expand/Edit Existing Module

[sqzr]

Hello

 

I am going to be expanding the Block Wishlist module. Its a rather simple change; adding a button and 1 function to the associate controller class.

 

What is the proper way to do this? If I go into the existing View and Controller files and edit them and then that module is updated - will my changes get overridded?

 

Instead of placing my code in the existing files should I simply include an external file?

 

Any advice would be appreciated.