It's not something built into Matlab, but it wouldn't be that difficult to code up. You could use some uicontrols to kick off a curve-picking routine.
That routine could grab the xdata and ydata from the curves' handles (use gco to pick them). Then you could use interp1 to interpolate one onto the other, do your addition and add the curve to the figure.
All in all, quite a neat little exercise to get you into handle graphics and uicontrols.