Using Sonar with other DAW'S

AFAIK all DAW’s have proprietary formats thus you can not load a Cubase track into Sonar, FL Studio or visa-versa.  However Wav and MIDI tracks can be freely transferred from one DAW to another.

Yes , you can if they are sending you audio only stems …or straight up midi where you supply the vst synths in the project
I do that all the time over here w different DAWs and software programs that I have over here on a few of my computers PC and Mac ….
it's pretty easy to do and there are a couple of things they could do on their end to make it easy for you ..
it helps greatly if you have the songs tempo , this way when you open a new project in SONAR you can set your project to your collab mates tempo and have things line up ..
it is generally a good idea for them to label the exports / stems for you so you can decide on how you would like to arrange the groups of tracks in your project .
ex drums w drums …Vox w Vox….ect act ..this makes it easy to work on the various sections as individual track groups ..
it's a very good idea for them to send you the files as a decent bit rate …ex a 24 bit wave undithered will give you a lot of headroom over at your end ...
make sure you can get the tracks somewhat dry w out a lot of effects on them ( unless they are must have efx that were printed during the recording process…ex …an overdriven gtr that was recorded w a must have tone )
make sure you have your Store Project in it's own Folder dialog box checked when you create the project ...
this way everything is together in one place in your saved project ...
a few of those suggestions are the absolute bare minimum for keeping your sanity during a collab that involves multiple DAWs ….
btw …I happen to enjoy that type of thing ….I have found that every DAW it's own thing going on w what it brings to the table 
 
Kenny

Yes , I agree with bitflipper on having the plain WAV's  , also on his point of having everything start at zero
 regardless of how much silence there might be before there is an actual musical part.
 
to augment what is being said here , may I suggest that you also have your collaborator  send you a reference mix of what his project sounds like exported from his DAW .
this is very important and the mix itself is not the main thing here ....
the exported mix is for your reference musically and visually .
you drop the reference track / mix  into your SONAR project , this will help you greatly with keeping your time line in order ...
you can see where all the major activity is for the various sections of the song
the other added benefit is for all the tracks that have a lot of silences in places  ....
with those you will now have a reference point to do your slip edits , and you can also correct any drift that might occurred . 
personally I like to do "apply trimming " to those edits  . it helps with the disc streaming and there is a lot less CPU resources being wasted that way .
to get everything lined up sonically I like to  play the reference mix while soloing each track individually 
I can't stress how often I've seen a difference between the reference track and the individual WAV files
it is easy to see where the phrase starts and how it ends ...if you have a phrase w a long sustained note that decreases in amplitude and then another phrase starts ...you will have a nice visual cue to line things up
I bring all of this up because it is very easy to correct this and it can be one of two things ....
either the reference track is not dead on in time , or the exported files are not dead on in time ....
to find out if the reference track is in time you solo it with the metronome playing along ....
do the same with the exported tracks to see if there is a difference .....
now here's where I'm different than most people ....if the stems all line up and play in time I go with that
my reasons are many ...some folks just cant seem to play in time , and sometimes unintentional edits mysteriously happen on these reference tracks .....LOL
the cool thing is
usually when people export individual tracks at a consistent uniform sample rate they will line up correctly in another DAW whether you know the tempo or not.
 
also it is a lot easier to move one track ,  the reference track ...( so everything  lines up )
if you are doing your job right the reference track will be muted most of the time any way ....
it's only there for reference ...
one thing I love is when I A/ B the parts I have added in the main project to the reference track
I know I'm getting real close when the main project starts sounding really good and the reference track sounds like lukewarm left over Dog CR*P lol.....
trust me when that happens you don't even have to tell people you use SONAR ....They Know ...
 
all the best ,
 
Kenny