i've had a look and you are just going to have to wait until renoise has rewire capabilities. unless you re-import cubase pieces back into renoise and duck there...

also an external mixer with a compressor and a kick track sent to it to duck with provide a basic vision of the sound you want before you export and move into cubase...

after all - hardware ducking is where it started...!!! :) The ever shit behringer composer pro mdx2200 will do this job and isnt expensive.