upgrade network card

I have a librebooted x200 which can only connect to 2,4 GHz networks. If I an not wrong this is due to the wireless card being N300 which. I also have what I believe is an N450 wireless card (AR5BXB112), which also works with 5GHz networks.

Can the N300 network card be upgraded to an N450 network card in the x200?

If so, can one add an extra antenna? The N300 card only has sockets for two antennas, and accordingly the x200 only has two antennas.

Can an extra antenna be added?