If you refer to plotconfusion() from the Neural Network toolbox, then unfortunately No. I just checked the code for it: if there is a "current figure" then it always clears the entire figure with clf(); if there is no "current figure" then it always creates a new figure. There is no direct way to subplot() it.
What you might be able to do is ensure there is no current figure, then let it generate a new figure, and afterwards go into the figure and extract the parts of interest and move them into a subplot in the place you want, and then delete that figure that it generated.
