I was wondering if its possible to add a proximity alarm to an E60 like what Viper alarms have as an addon or standalone unit in addition to our alarm?

 

Without looking into it in detail, a proxy sensor operates by showing a ground when its EM field is disturbed. You could purchase a DEI stand alone proxy sensor and install it like a shock sensor as seen in this DIY:

https://5series.net/forums/topic/109...o-stock-alarm/

The car itself has a motion sensor but it is limited to the interior of the vehicle. If you want something that would detect outside of the interior, you would have to purchase an aftermarket unit.