The width of the submix is specified in the destination track where the submix is used. When you set the input to a track and specify a submix, there is an option called: Submix Input Channels where you can select from 1 to 8 input channels.
If you have mono processing Sounds in the destination track, you may also want to force those to be stereo (or multichannel) to match the number of input channels. To set the number of output channel, use that same menu and choose: Track output channels (which can be the same as the Sound or you can set it to 1 to 8 channels).
In the Kyma 7 folder, there's a folder with Multigrid examples; one of them is called Four channel submix processing example.mgd, and it's an illustration of how to send multichannel submixes between tracks, process them, and then mix down to stereo.