Hi,
Like RN-171<->USART-(16F729)-SPI 18F4550 you mean?
If you think it'll be easier than changing the PIC then fine. You may need to buffer the data in the 16F729 if you're in the middle of a SPI transfer with another device. What are you using the USB for? Perhaps you could you use a PIC with a MSSP module (SPI), built in CAN and 2 USARTS. Use one USART for the RN-171 and the other for FTDI USB chip or simmilar? (PIC18F45K80 is an example). Honestly, I don't know enough about your project so the only thing I can do is throw ideas up in the air and at this point I'm running out of them.

/Henrik.