Does anyone know whether it is it possible to enable it due to software / firmware upgrade or other kind of upgrade and how to do it?
It's not possible to "unlock" the N-function. To achieve N-functionality, you need to swap the wifi-card with another, and (if you don't already have the third antenna) install a new set of antenna-cables. Most T61 and later systems shipped with 3 antenae wires - black, grey and white. The black and grey are used on 2 antenae connection cards with the white wire capped and tucked underneath the card. By removing the keyboard, you can quickly determine whether your system as 2 or 3 wires present. The blue and red wires (if also present) are for the WWAN function.
Note that if you don't get an approved card from IBM/Lenovo, your BIOS will throw an error (unless you run a modified BIOS without a whitelist present).