Wait, I think I am little by little understanding what you mean... please tell me if I am wrong
You mean, to use as CS pins the IOC pins of the master (in my case RB4~RB7), and instead of setting them as output all the time (as I do know with TRISB=0), setting them as input (TRISB=1). (That would mean tristate the pins). Obviously with that configuration they can't be used as CS so whenever I have to use them as CS, only then put them as output (TRIS=0) , select the slave, transmit, receive and when finish, not only unselect the chip (by putting CS=1) but also turn them again to input (TRIS=1).
Meanwhile, when the slave finish and needs service, it will send some signal through those lines and since CS is input, the master will sense that thorugh IOC interruption and service it (by doing what I wrote above)
Is that what you meant?
If this is so, the only issue I still got is
what happen to the SS pin on the slave? If we have configured it with SSPM3~0 as 0100 then can we suddenly use this pin as output as well???