Can I use the 32.768 kHz output from a RTC instead of an external oscillator on a PIC MCU?
Sure you can, but why not use the internal oscillator of the PIC?  There are a few different modes of it, and its accurate enough for most situations.

A couple of things to remember if you use the 32kHz output of the RTC.  Make sure its wired correctly to the PIC clock input.  I believe it gets connected CLKIN, and then CLKOUT gets left floating or needs to be pulled some direction, I don't remember off the top of my head.  The RTC datasheet indicates that the 32kHz is active by default.  If that ever accidentally gets turned off, there goes the whole neighborhood as now the PIC has no clock, and cannot talk to the RTC and turn it back on.

Addition: As Nick said, the clock output of the RTC is an open drain.  You will need to add a pull-up resistor to to CLKIN to ensure correct functionality.