Greetings; There are plenty of VST 2.4 plugins that can do side-chaining in Cubase, such as the Sonalksis stuff — i double checked the process before i replied, to make sure i wasn't tripping, and i'll be happy to send screen shots to show how it's done.

A discussion of the process can be found at http://www.gearslutz.com/board/music-computers/359606-list-vst-compressors-allow-quad-input-sidechain.html,, or better still check out http://www.native-instruments.com/knowledge/questions/984/How+to+Create+a+Sidechain+Effect+in+Cubase (not sure that the HTML links are working for me in this forum, s'ry about the long URLs).

It would be awesome to have this functionality available in Omnipressor, and H3000 would be better still — side-chaining mods patches would be off the charts on this plugin!

Regards
Glenn