[TYPO3-dev] Howto extend 3rd partxy Extension?

[Dirk Ho]

Hi,

before I start coding, I wanted to ask, how best practice is to extend a 
3rd party extension - in my case it's Powermail2.

I want to add functionality to add a PDF with all user data to the 
double opt-in mail, if the user sending the form is still under 18.

I first thought about to extend the class that contains the methods I 
want to override and symlink all files of powermail2 to my extension, 
but that makes no sense at all. If this would be the solution I could 
also add my code directly to the powermail2 extension - it wouldn't be 
updateable anymore.

Could you please help me, how to do this? I'm using Typo3 6.2.x and 
Powermail 2.

Because I use powermail2 not only for a membership form, but also for a 
contact form, I need the possibility to set a flag, whether to use the 
new functionality or not.

Thanks and best wishes,

Dirk