audio recording and video editing software
I don't know Pitivi but in general it's not hard to do what you need, just sometimes a little time consuming. Use a digital audio recorder that can record 48K at 16 bits at a solid fixed rate.

In this case since you already have sync audio from the camera, you can use it as a reference track to find and check sync with the separate track. It helps to use a clapper at the beginning and end of each scene, but since you have both tracks you might not need a visual clap, just a recognizable audio clap. If possible announce the scene and take so that it's audible on both.

If the sep sound drifts out of sync you can use those start and end claps to find the exact frame duration and adjust the rate of the sep sound to match. If Pitivi does this, great, but you can also use a free tool like Audacity.