[support] Extending existing module

[Afan Pasalic]

Hi,
This time I didn’t do anything embarrassing. :-)

How can I extend an existing module? E.g. I want to add some features/validations in web form module. I know I MUST NOT change the code in the webform module itself. 
I believe there is some solution to extend a module. Something like extending a theme (sub-theme).  And updating the module will not break the website.