SPI communication with multiple slaves

First, you have identified a really annoying part of SPI on the AVR: Connecting SPI slaves will interfere with the proper operation of ICSP.

Second: The SPI bus has MISO, MOSI, and a chip select pin. You can break out the MISO, MOSI and GND pins, and perhaps VCC, plus a different CS pin for each daughter board you want to connect. As long as the CS is not active for the daughter boards, and the daughter board chips do not try to use the bus in master mode, you should be OK.

Third: You can use a buffer chip of some sort (perhaps 74HC125 or similar) with a tri-state function to isolate the ICSP feature from the SPI-talking-to-slaves feature; set the buffer chip up to require a particular pin high to let signals through, and use software to pull that pin high only when your code wants to talk to the slaves, and keep it low after a code reset.

From the CC2520 datasheet it looks like all you need to do is control the resetn of the CC2520 with any GPIO pin. As long as you can drive the pin high and low you should be fine. Any of those pins should work provided they are configured as outputs.

I might consider adding an external pullup resistor on that resetn line if you don't want the CC2520 to be reset when the MCU gets reset, otherwise it shouldn't really matter.