Should be doable as long as you have the non-master key for that lock too. A standard key lock works by having pins which are separated into two pieces. The key lines up the boundaries with the cylinder, which allows the cylinder to turn. Master key systems are typically done by having the pins be separated into <i>three</i> pieces, which allows two positions to turn the cylinder. One position is used by the normal key, and one is used by the master key. Having the normal key will let you figure out which is which, and then you can figure out the master key from that.