I believe you need a scan tool to do it, maybe someone knows a way to put the system into program mode without one. Regardless, here'* what the 2000 Service Manual says about it:
Important : All Transmitters which are to be recognized by the remote function actuator module must be programmed in a single programming sequence. If the system is placed in program mode, all previously programmed transmitters will be erased upon the receipt of the programming signal from the first transmitter. Up to four transmitters may be programmed. Do not operate or program the transmitters in the vicinity of other vehicles that are in the keyless entry program mode. This prevents the programming of the transmitters to the incorrect vehicle. The order in which the transmitters are programmed is important. The first transmitter programmed will be transmitter #1, and the second transmitter to be programmed will be transmitter #2. The number on the transmitters are for reference only: #2 can be programmed as #1, or vice versa. Additional unnumbered transmitters are also available. Use care to program the transmitters correctly.
1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Use the scan tool RFA Module Special Function: Program Key Fobs in order to place the RFA module into Program Mode. The doors will lock and then unlock to confirm program mode.
Important : The scan tool must stay connected until all of the transmitters are programmed.
4. Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of 15 seconds, the doors will lock and unlock in order to confirm the programming of that transmitter.
5. Repeat the previous step in order to program up to 4 transmitters.
6. Remove the scan tool.
7. Operate the transmitter functions in order to verify correct system operation.