The BMW/Alpine system is supposed to have glass-break protection, there should be a mic right next to the warning LED, or on the underside of the alarm brain depending on which one you have (pre or post "1995" pre '95 have the rectangle 3 button remote with a removeable coded chip, the post 95 ones have a round 2 button remote with a code built in). If you open up the brain on either, you can adjust the sensitivity, mine was set way too insensitive (braking glass bottles inside the car (into a bucket of course) wouldn't set it off before, but now just tapping on a window with a washer sets it off)
The best way that I can think of to add another sensor would only allow a full trigger of the alarm. The BMW/Alpine system has an input for the hood sensor. It is the single pin plug that comes off of the body and alarm brain harness. I think it is a white/red striped wire. If you look at pictures for an OEM alarm install you should see it. Anyway, pick a sensor that you can adapt the trigger output on, and splice ( and solder it correctly) into the hood sensor wire. Isolate the two sensors with diodes, one on each wire from the sensor before they meet at the splice, and a trigger from either the hood sensor or the new one should set it off.
I don't know exactly how the hood sensor triggers the alarm (a -ground, +voltage, or a combination of +voltages or how much). I am assuming you have some electrical knowledge if you are screwing around inside of your BMW, and know what a diode is and does. I think I have seen this somewhere else on the web in a write up, however, I would just use a voltmeter to test it out myself.
These are just ideas of mine, I take no responsibility for you screwing up your own BMW blah blah blah.