Link to the camcorder manual
I found no reference to adding your own music.
Since this is "editing" I would not use the camcorder to edit - but would import the video to the computer and edit there. And you can add your own music, too.
Connect the camcorder to the Mac with a USB cable, put the camcorder into PC mode (see page 137) - and assuming your iMac is running a current version of iMovie, you can Import or Capture the video from the camcorder into iMovie... Edit, add transitions, audio track, remove shakiness, etc... then export to whatever file you need for whatever audience is required...
But yes, compared to audio dubbing the way you used to do, I cannot find an equivalent in the manual.
There is a LOT more to all this, but the short version seemed to be the right place to start.
Having used a Panasonic 180 NV camcorder for 4 years, dubbing music onto a different part of the tape but retaining the video was as easy as it could be.How do you do the same with an SD card without involving the computer. I run an iMac 10.6.8 for info. It would appear to be much more complicated.