If it has power windows and locks, you can add it. Im actually surprised it doesnt have it...almost all classic trucks with power windows and power locks come with remote keyless entry by default.
Since you just bought it, Id be willing to bet it actually does
have keyless entry, and the previous owner simply just lost all the remotes.......
Look at the glovebox RPO sticker and see if you see the following codes: "AU0" "A31" and "AU3". If you have all three of those codes listed, then your truck already has remote keyless entry and all you have to do is buy a new remote and follow the programming procedure that I listed below...
If it doesnt ahve all three of those codes listed, that means it doesnt have keyless entry, BUT you can add it.......to add it, you'll need to buy the remotes (you can find them on ebay for like $20, make sure you get the proper remote though, for a 2003-2007 pickup), and you might need to have the dealer reprogram/reflash the BCM (body control module) with a Tech 2 to add/activate the keyless entry feature. Im not 100% sure it will need to be programmed though, it might work without being reprogrammed? To program the actual remotes themselves, follow these steps:
1. Close all the vehicle doors.
2. Insert the ignition key into the ignition lock cylinder.
3. Press and hold the door unlock switch.
4. While holding the door lock switch in the unlock position, cycle the ignition ON, OFF, ON, OFF.
5. Release the door unlock switch. The doors will lock and unlock to confirm the program mode.
6. Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of approximately 15 seconds, the doors will lock and unlock to confirm the programming of that transmitter.
7. Repeat the previous step to program up to 4 transmitters.
8. Turn the ignition switch to the RUN position in order to exit the keyless entry transmitter programming mode.
9. Operate the transmitter functions in order to verify correct system operation.
SO basically what I would do is...buy a remote on ebay. Try the programming procedure I posted above. If it doesnt work the first time, try it again. Then if it still doesnt work, you'll have to go to the dealer and have them reflash the BCM.
Depending how nice your dealer is, they might do it for free, or they might charge you the minimum allowable labor time (usually 1 hour or something, which kinda sucks because the actual reflash procedure only takes like 5 minutes)