Hi again,
I found a solution to my problem so I'd like to share it if s.o else need it 
I found a software implementation of the TWI module of arduino in the atmel avr site. This driver emulates the hardware TWI module of ATmega. It uses two pins for SDA and SCL that you can modifiy and the control over slave address transmitted by the master is transparent. This driver allows using I2C on low cost devices that doesn't have TWI neither USI modules or on sophisticated microcontrollers when you need to put your hand over details, control everything and be hardware independant (which is my case).
Here is the link to the application note Atmel AVR154: Software Emulation of TWI Slave Hardware Module http://www.atmel.com/Images/doc8478.pdf
The soft is available here http://www.atmel.com/devices/atxmega64a1u.aspx?tab=documents.