1998 Chevy Silverado keyfob

Yes, but you will need to program all of them at the same time.

Remote Control Door Lock Transmitter Programming
If they key is in the ON position during the programming procedure, the horn chirp will be disabled, if the key is in the OFF position, the horn chirp will be enabled. The horn will chirp when the lock button is pressed two times.

When reprogramming the remote control door lock reciever, All transmitters must be programmed at this time in order for the transmitters to operate the vehicle. Transmitters not programed at this time will not work and the procedure will have to be redone with all the transmitters.

"Â Ground pin 4 to pin 8 of the data link connector (DLC).
The receiver will respond by locking and unlocking the doors, and activating the liftgate window release (if equipped) within 2 seconds.

"Â Press and hold the LOCK and UNLOCK buttons simultaneously on the first transmitter to be programmed until the locks cycle (approximately 10 seconds).
"Â Repeat step 2 for up to four total transmitters to be programmed (if available).
The receiver will respond by locking and unlocking the doors and activating the liftgate window release (if equipped) within 2 seconds.

"Â Remove the ground applied to the DLC.
"Â Lock and unlock the doors in order to verify the transmitter operation.
"Â Activate the rear window release in order to verify the transmitter operation.

Yes, it is the diagnostic connector under the driver side of the dash. If you remove the two screws that hold the connector to the dash, you will be able to see numbers on the connector housing that identify the terminals.