The ICSP uses SPI (MISO, MOSI, SCK) and these are also connected to the Esplora 10 pin expansion header. XBees and similar radios communicate over TTL serial (RX, TX). You are right that the Seeed documentation is rather poor.
I did use the SoftwareSerial library to connect to the XBee (in addition to the Serial1 hardware connection) and it works as expected. So one could repurpose two of the pins in the ICSP header to do TTL serial communications, yes. The Seeed code uses pins 11 and 12 and NewSoftSerial and that should work... But you most likely could not also do display on the new expansion header as this uses these SPI pins in their intended configuration.
A better option could be to use D3 and D11 exposed on the two orange TinkerKit connectors with SoftSerial. These would not interfere with the display or SPI functionality. There is not a pre-made board for this though.
Keep up working on possibilities.