Sure. In the lower right hand corner of the VST window where you open your VST, there is a box called "midi channel" in which you can choose "all" and then set various midi channels on multi-timbral products or you can choose a specific channel for the VST.
Good to know this, but in the case of Spectrasonics Atmosphere: I just don't see it. For Kompaxt and Intakt instruments I can set it in the Kompakt and Intakt windows after pressing "e" in VST Instruments window in VSTack.
But for Atmosphere I can't see where I can set the midi channel, only the midi port.