I'm not saying there isn't a way to make it work but I'm pretty sure PBP doesn't support it natively.
You'd need to somehow fool the compiler into thinking the device has more memory than it has (if that's even possible). Then the compiler and assembler generates a single .hex file which you'd need to split at the correct location, program the first half into the PIC itself and the second half into the external memory device using some other type of device programmer.

I don't know the format of the .hex file, if there's any sort of header and/or checksum or anything like that. If there is, then simply cutting the file into two most likely isn't going to work and some sort of software would need to be written to repackage the large hex into two with the correct format.

Again, I'm not saying it isn't possible and I look forward to hear about any experiments. All I'm saying is that I don't think it's going to be straight forward.

/Henrik.