That’s not a big problem. People do it all the time. Usually we have the same
timecode associated with both the video and the audio, but you can still do it and
sync up manually.
Load in both the video and the audio you don’t like from your DV camera.
(use that as a reference for sync).
Then load in your external audio. (I’m not sure how you plan to do that
since you didn’t mention the device you will use to record the audio).
Now bring them both onto a timeline and while listening to all the audio
tracks, slide one or the other around until they are synced.
Pretty simple once you do it a few times.
After you get them in sync, simply delete the audio tracks you don’t want to use.
It gets complicated when you have many takes and the camera starts and stops,
but if this is what you want to do, that’s how you do it. Of course, if you can
use a timecode generator and feed both the camera and a hard drive audio
recorder with the same timecode, then you do what a lot of us do on shoots
to protect our audio feed. But that costs more, obviously.
Dan