Silent monitoring is possible using the ChanSpy set of features (ChanSpy - 555 - for random channels, ChanSpy Direct - 779 - for a specific extension, and ChanWhisper Direct - 778 - for a specific extension with the whisper capability).
I am not aware of an option to barge in. Due to that, for example to get his/her supervisor involved in the call, a CC agent has to use the conference feature.