I am using the felogin system extensions. Those extensions are part of the default TYPO3 installation and they are stored in typo3/sysext. I know that I can edit them by editing its code directly but I guess it is not the best way of doing it.
How can I “overwrite” those extensions without changing their code?
Shall I create a new custom plugin and copy & paste code of a system plugin I wish to alter?
You could just modify the extension and place a copy in typo3conf/ext/. This directory (so called “local extensions”) override the system extensions.
However, you will probably run into trouble when you want to upgrade TYPO3.
Did you find a bug, if so, please report it. If you know, how to fix, then it would be great, if you would push the change into the review system
If you just want to change functionality, modifying existing code is not a really could solution. There’s the concept of hooks (which is a clean one) and of XLCASSes, which is somehow dirty (as it can also cause trouble with newer TYPO3 versions). So basically the same as with every other extension.
Steffen