Grab Runaway's offer with both hands!
However, for the future, I offer Uncle Bobbsy's cheating way of extending a track if you're not confident with cutting and pasting.
Use "Copy to New" to make a new copy of the backing track and insert it into multitrack. Slide the new track to the right so it "sticks out" by roughly the verse and chorus you want. Use volume envelopes to mute the first part of the track (before the "extra" verse and chorus). Assuming the song is fairly repetitive, leave a bit of extra un-muted before the transition you want.
Move the CTI to the part of the tracks you're working on. Play the two together and slide the copy track so it's exactly in time with the original.
Use volume envelopes on both tracks to create a really fast cross fade at the point you want to move to the extra bit. Zoom in and use the CTI to line up the envelope points so the start and end of the fade up and fade down are exactly the same point.
You can either do a mix down right now to create a revised backing track or, if you're going to be mixing a bunch of stuff anyway, just use the two tracks as part of your mix.
Purists we say that you should just practice using the cut and paste until you're confident (and technically they're right) but the above is a quick and easy way to achieve the same thing.