You can program additional ChipKeys to your  2002 and up Mitsubishi using the following method. This procedure is in your owners manual.
This procedure will not work for year models 2000 or 2001.

    Customer Key Programming

    You can program new keys to the system if you have two valid
    (already registered) keys by doing the following:

1. Insert the first valid key into the ignition and turn the ignition
    to the "ON" position for 5 seconds.

2. Turn the ignition to the "OFF" position and remove the first key.

3. Insert the second valid key and switch the ignition to the "ON"
    position within 7 seconds. Theft-alarm indicator will blink twice.
    Turn the ignition to the "OFF" position and remove the second
    key.

4. Insert a blank (transponder) key, specially cut for your vehicle 
    into the ignition and switch the ignition to the "ON" position
    within 7 seconds.
    The theft-alarm indicator will come on for about 3 seconds when
    the registration is completed.
    If an error occurs, the indicator will blink 3 times.

5. If you want to register additional keys, repeat step 4 within 30
    seconds after the theft-alarm indicator flashes in step 3.

    Note:
    You are provided with 2 keys, but you may add up to 6 more keys.

   If you only have one key or a 2000 or 2001 model and need a locksmith
   to help you click here to find one in your area.