Squibcakes, All of the pins on the Pic's that I have seen have reversed biased protection diodes from VSS to VDD. If you were to limit the current of the -15 volts signal and connect it to one of the processor pins it would place that pin at around -.7 volts in reference to ground. This is not a good practice and a great way of destroying a good Pic processor. Have a look at using an optoIsolator to do the interface.

Dave Purola,
N8NTA