Membrane keypad would have 2 issues that would prevent it, first the membrane is most likely not heat resistant like flexable PCBs and would melt if you tried to solder to it, second the black traces on the membrane are made of powdered carbon in a binder and would not take the solder or would break apart. You could use a pressure fit to hold the contact of the switch to the traces if you where somehow able to hold the switch in place.