Merging several songs into one?

My "search-fu" has failed me on this. Maybe someone knows an easy way to do this. I have a client that recorded 6 songs with me, all in their own project files. Now he has decided that he wants to combine them, along with some sound effects & additional instrumentation to make one long, Pink Floyd-ish super song.
 
All of the songs use the same template, and the tracks are quite simple: two tracks of acoustic guitar (neck & sound-hole), one track of vocals, one "room" or "ambience" track. Most of them are a single take, so with those I could just import the audio & align them to where he wants them. A couple are comped, so at this point I'm thinking to just bounce the tracks & import them. He wants me to mix them individually, then combine the stereo mixdowns together with the new stuff to make the "super song," but I have reservations about that. I'd like to be able to tweak the individual tracks in the "super song" because in some places we're adding strings, & other places get piano, & yet others get a little percussion, and he wants to tie some of them together with the additional instrumentation. So keeping everything separate seems smart to me, so we can tweak panning, automation, etc, as we go.
 
So the question is: is there a way to import or merge multiple songs? Or (since the track count is so low) should I just fly the audio files in and be done with it?
 
Thanks for any suggestions!
Clark
SONAR Platinum, Win10 x64, i5 quad-core, 16GB RAM,  Focusrite Scarlet 18i20g2, other stuff to boring to list

bitflipper
Export the six (unmastered) songs to 32-bit wave files, then import those 6 files into a new project. They can all be on the same stereo track for ease of crossfading. If the songs differ in levels, put volume automation on the track so they can be level-matched. Put your new audio/effects on separate tracks and mix them like you would do normally. 

What he said.

To add to the above. if after assembling the "super-song" you decide a song needs be tweaked, just do a new mix and replace the existing file with the new one.
 
However, note that you can open up another window (as long as "Allow Only One Open Project at a Time" in Preferences is unchecked), and then drag all the files over to the original song. Given that you're using a template, that should make life easier as the tracks will all line up properly.