This is what I do if I want to add another track to an existing song:
Insert the song into the multitrack (Track 1).
*Open "Edit", then "Insert Blank Wave". This should appear in the multitrack as Track 2.
Arm Track 2 to record.
Highlight an area in Track 2 that is close to where you'd like to add the extra part to Track 1, and extend that highlight on out for a few minutes, or at least long enough to contain the new part you want to add.
Hit the "loop" button. This is important!
Hit record, and the cursor will begin from the beginning of the looped point. Record the part you want, and then stop recording. Hopefully, you extended the highlighted area out far enough to contain the new part you want. Otherwise, it'll loop back around and record over everything from the beginning!
*You don't REALLY need to insert a blank wave - it'll work just fine if you just highlight the area you're going to record in - but it makes things easier should you happen to want to add something in ahead of the part you're puting in now. If you decide not to insert a blank wav, be sure you save the session. If you don't insert a blank wav, accidentally right-clicking on the added part and moving your cursor will shift the whole wav out of synch with Track 1, and you'll have a tough time lining it up again.
Hope this helps.