Combining Left and Right Hook Speaker Wires Into One Speaker
Shorting the wires is not a good idea.  Each signal is produced by a separate amplifier, which is probably using feedback to make sure it is the voltage it is supposed to be.  When you short the two together, the two ampliers will be fighting each other.

To be safe, put a 4 &Omega; resistor in series with each lead before shorting them together as you describe.  That is a total of 4 resistors, one in series with each of the + and - outputs for each side.  Then connect the single speaker between the two resistors coming from the + outputs on one side, and the other side of the speaker to the two resistors coming from the - outputs.  The speaker will then effectively emit the average of the two stereo signals, which is basically the mono signals.

There are other ways this could be done, but without any specific information about the circuit, this is the conservative approach.  It will waste a little power, but if you really care, get the right speakers.