Hi, i'm using RC522 with an Nano, <AddicoreRFID.h> library, used only to read serial number (4 bytes)
And i'm wondering if i can unplug some pins from the Nano in order to use them for something else, 6 pins are too much for me
No you can't.
straight question, straight answer 
But i really need to, at least one, the reset pin from the RFID and the SDA pin too, they are only used at the setup i think
pinMode(chipSelectPin,OUTPUT); // Set digital pin 10 as OUTPUT to connect it to the RFID /ENABLE pin
digitalWrite(chipSelectPin, LOW); // Activate the RFID reader
pinMode(NRSTPD,OUTPUT); // Set digital pin 10 , Not Reset and Power-down
digitalWrite(NRSTPD, HIGH);
oh, ok then tnx
Do you know you can use the first six analogue pins as digital ones.