Good Evening,
Not to hard if you are electronically inclined. Purchase a low voltage (DC coil) 6 pole double throw relay, if shielded all the better. Open up the stereo and find your volume control. It probably has three wires on each section, one for the right, the second for the left. Place the wires going to the pots on the common terminal of the relay(s). Place the front panel pots on the NC (Normally Closed) contacts and add another pot to the NO (Normally Open) contacts. I then covered the relay with a grounded piece of copper foil. Wire the relay through a connector if you like and connect it to an appropriate DC output plug in power supply (less chance of hum). 100 Ma output would probably be good enough, more will not hurt but check the relay coil current to be sure. Put it back together hook up the speakers etc. Turn it on and it should work normally. Plug in the power supply and it should switch to the new volume controls you installed. Adjust to the alarm volume and you are all set. Mine is adjustable through a cooling slot in the top of the cabinet. Unplug the power supply and in a few seconds it should revert back to the front panel volume control.
Enjoy
Gil Shultz