I am completeky new to the tool and would like to find out if I can first record some narration segments independently (i.e. not directly while using Camtasia) and then add them segment by segment by putting them in the places I want them to be?
Yes you can. You can import the audio file and place it on a track in the timeline. From there you can split the audio as often as you want. Then just position the segment wherever you want. It might be easier place the main audio on one track and then drag the audio segments to another track. That way your segments placement cannot be impeded by the other audio track.