How To Extract A Loop From A Song
You can extract a portion of any song and make it into a loop.  This loop can then be combined with other loops or beat matched songs.

If the song is in time with the metronome and is beat matched properly, you can extract a seamless loop. You may need to fix the song if the tempo is drifting.  Make sure that snap is on and aligned to the measure or bar.  

Select the measure or measures of audio on the sound and click Copy from the Edit menu (Ctrl+C).  Paste it on a new track by clicking Paste from the Edit menu (Ctrl+V).  Select the new click and then click Make Into A Loop from the Sound menu (or right click on the sound and select Make Into A Loop).  Loop the new sound as many times as you'd like!