People do this sort of thing all the time. The easiest way is to render each track as a
.wav or .aiff file that starts at the same point for each track. Most software will do
this fairly automatically but I'm not sure of the exact commands needed for Cubase. You
can then load up these files into a new session in Sonar.
You may also find
that Sonar can accept a file format that Cubase can export (OMF??). Alternatively you can
use a program like Pro Convert or AATranslator to convert the sessions - although neither
JRP Music - Audio Mastering and Restoration.