Most often, there is no need to convert individual ReWire channels to regular audio tracks. The channels already appear in the host application’s Mixer, and you can typically perform the same kind of real-time processing as with regular audio channels (effects, EQ, volume, pan and mute automation, etc.).
Still, you may need to convert the ReWire channels to audio tracks, for example if you want to continue working in Cubase only. This is probably easiest done by using Reason Essentials’s internal “Bounce Mixer Channels” function, described in
“Bouncing Mixer Channels”. When you have bounced the desired Mixer Channels, you could then import the generated audio files to tracks in the host application for further processing.