Is there such a thing as pluggable twisted apps some one can add to a twisted project? One example of this could be a web logging. That's a web app where you can access the results of your logging process.
There's no magic dust sprinkled over the whole of Twisted which will let you insert arbitrary hooks into arbitrary places (Or... there is, it's called "monkey patching").
Specific interfaces have specific ways in which you can extend them, though. For example, the logging system is based on events which are emitted and a list of observers to which those events are delivered. You can add new observers with whatever behavior you like.
Each such point of extension needs to be dealt with in its own way. So first identify the thing you want to extend, then look for its documentation (or, if necessary, read its implementation), then use what you've learned to extend it. Or file an enhancement request if the behavior is not customizable in the way you want.