not inside nebula. you have to do it via plugins like the one mentioned above. you could, for example, split the signal into mid and side signal on two tracks and give them seperate effects via two nebula instances.
system 1: windows 8 32 bit - samplitude prox/x3, tracktion6/7, reaper system 2: mac osx yosemite - reaper(32+64bit), tracktion6/7(32+64bit)
both systems on: macbook pro (late 2009), core 2 duo 3,06 ghz, 4 gb ram, graphic: nvidia geforce 9600M GT 512 MB
yeah the mid/side conversion itself can't be done by nebula. you could do the conversion to mid/side first then process with a nebula program, then go back to l/r, as people have mentioned, but any kind of hardware that converts the input to mid/side then does processing then converts back to l/r can't be sampled properly. so like any kind of stereo tools that expand the width with mid/side processing- that can't be sampled. you could sample it, but it wouldn't recreate the effect of the stereo widener because that would require nebula to somehow process the stereo sum and difference separately and it only works on l/r inputs.