Is there any software i2c library that allows arduino to behave like slave?, haven't found any yet
I found this : http://www.atmel.com/Images/doc8478.pdf
a library that emulates the hardware TWI module on arduino which checks the address sent by the master.
the software is here : http://www.atmel.com/Images/doc8478.pdf
you need to adapt it (the driver) to your board : change the pins numbers etc.
You also need to compile it with IAR workbench IDE then modify it to change your board behavior according to the slave address.
After that I don't know if you need to use Wire library to communicates because the library is for low level I2C. So maybe you'll need after to adapt your Wire library to the Soft TWI one.
It doesn't seem very simple and let us know if you make it.
Good luck.