You can use a MIDI device generating MIDI System Realtime messages to control KMetronome. System Realtime messages accepted are: Start (0xFA), Stop (0xFC) and Continue (0xFB).
Many master MIDI keyboards and MIDI buttons devices provide those three MIDI transport controls. You need to connect your external MIDI devices to the computer using MIDI (or USB) cables, and connect the corresponding ALSA port to the input port of KMetronome. This connection must be done by hand, or using a session management software like QJackCtl.