This can be done in a simple way by mounting a male header and plug in the unterminated "AF In" connector (J5). The lower pin on the header should be soldered to the warm speaker cable. See middle right in the image (press image to zoom in).
If the KPA100 is mounted in the K2, J5 is plugged into the K2 main board, and when it is used stand alone, J5 may be plugged into the new header. In this way the KPA100 loudspeaker connection is compatible with the original design when mounted in the K2, and its loudspeaker can be used for other rigs when the KPA100 is separate from the K2.
Be sure to measure the resistance at the end of a mini-jack cable plugged into the external speaker connector before trying the loudspeaker. It should measure about 4 ohms, and if it is less than 1 ohm you have made a short to ground, so check the header once more.