You have to bear in mind two factors:
1. headphones (at least normal dynamic ones) need only a small voltage to drive them compared to loudspeakers.
2. many amplifiers get unhappy if you run them without any load (no loudspeakers).
So some designs use headphone sockets that carry the signal to the loudspeaker through them and a mechanical contact isolates the speaker wire as a plug is inserted, generally the sound is then diverted to the headphone contact through a resistor to limit the volume, this is scenario 1 above.
Some, including yours if that is the correct schematic, will croak if the loudspeaker is disconnected (as you can't hear anything the first sign of trouble is smoke!). In that scenario, when the headphone is plugged in, the main amplifier has to be shut down to prevent damage. The other factor here is that some smaller amplifier has to take over just to drive the headphones.
Shorting those contacts will probably work but test it before turning the power on. Measure the resistance across CL1185 and CL1187 with and without headphone plugged in and make sure it really does work as the schematic suggests. Be sure the resistance goes from high to almost zero Ohms as the switch operates, there is a danger of measuring the headphone resistance which is usually only a few Ohms but still higher than a complete short. If the switch does indeed swap the output from speakers to headphones it should be safe to wire an external switch to do the same thing.
The alternative method is to take the signal from the speaker connections all the time and use a changeover switch to divert the signal externally from the unit. You need to use some power rated resistors to fool the amplifier into thinking the speakers are still connected and some dropper resistors to limit the headphone volume. I would not recommend this method if the amplifier regularly runs at high power (> 25W) as the switch contacts and resistor ratings will make it impractical to build.
Brian.