Although it is possible to do what you say (if you pick your RECEIVE pin on Port B), I really wouldn't use SERIN/SEROUT. If your device has a hardware serial port - use it. Interrupts are far, far simpler that way.

If you have no hardware serial port, and you absolutely must have an interrupt, I wrote some code 2-3 years ago that gets the job done. It might take me awhile to find it, though.

But do you best to use the hardware first....