Is there any software i2c library that allows arduino to behave like slave?, haven't found any yet
I've found this http://www.atmel.com/Images/doc8478.pdf
It is a software emulation of the hardware TWI module on Arduino which checks the slave address during communication.
You can find the software in the AVR site.
However, you'll need to adapt the driver ot your board : change the pin numbers etc and also change the behavior of your board according to the slave address.
You can compile it only with IAR workbench IDE and maybe you'll have to adapt your wire library too. I think they should be used together (Wire and soft TWI libraries). This is not very simple and not sure if it's gonna work.
Good luck and let us know if you make it.