I'm sure it would be possible.
http://www.ebay.co.uk/itm/USB-RFID-Read ... 1704wt_934
that basically acts as a keyboard and the 'tag' just inputs a 10 digit code so the plan would be hook this up to the Raspberry Pi then have a piece of Code where if it detects those numbers as input sends the signal to release the door lock. i imagine this would be a simple piece of code?
i wouldn't know where to begin with that bit of code though, or weather the Raspberry Pi could output the voltages a Mag requires 12/24v