[Typo3] extending extensions

[jim]

Hi all,

I'm new to typo3 and was wondering if anyone knew of any 
documentation/links that would help explain the process of extending 
extensions.

For example i am looking to modify the feusers_admin extension to 
include/remove some extra fields.  I know there may be specific emails 
relating to this extension which i will look up/at, but I am curious 
about extending extensions in general, and don't know where to start.

Thanks

JIm