Does your version of cubase have ReWire? If it does then load your reason song, open cubase and add a ReWire channel. Hopefully if they're set up right, they will sync so when you press play in cubase, it plays reason along with it. This will mean you can still edit your reason tracks from scratch. You can also add effects and the like to your reason song. Also you can split up the reason instruments to different ReWire channels to play them all seperately for different effects etc.
If your version of cubase doesn't have ReWire, then i guess you are stuck with exporting as a .wav file and importing into cubase, sorry!
A warning though, cubase and reason are very intensive programs and running both will be extremely demanding.
I hope this is what you mean, if not then ignore...