You would have to solder a simple connector. Find 4 pin jack, solder needed contacts to jack that would be connected to the guitar amplifier and add a resistor (if needed) between ground and mic contacts.
You could also need a resistor between amplifier and n900 if amplifier's output is too high (I suspect it to be line-out which has bigger voltages than mic)
Look here for schematics:
http://kibergus.su/en/node/39