This isn't possible today. Can you explain more about what you want to do on the pool subset if you could create one? What do you want to run there, and how would it use multiple workers of the subset? One thing I can imagine is doing something like this:
p = parpool(..);
[subPool1, subPool2] = somehowDividePool(p);
fut = parfeval(subPool1, @myFunctionThatCallsParfor, numOuts, in1, in2);
Here, myFunctionThatCallsParfor has a parfor loop inside, and it gets to use the workers from subPool1 to run that.
Is that the sort of thing you're interested in? If not, it would be useful if you could explain what you would like to be able to do.
