convert keyless drill to keyed?

Sure, just buy a new chuck. Keep in mind, a crappy chuck is a crappy chuck no matter if it is keyed or not. There are some good keyless chucks available that probably cost more than the Skil brand drill.

Most chucks are held on by a bold with an allen head in the bottom. It is a reverse thread. Take that out first. Sometimes you have to put the allen wrench into (short end in) the chuck and rap it with a hammer to loosen it.

You can replace it but a good 1/2" might cost more than a new decent drill.

My recommendation too. The only keyless chuck drill I bought at a garage sale had an integral shaft to the motor drive. The chuck is not removeable from this shaft. My impression is drills are so cheap nowadays (essentially throwaways) that they might as well save on the manufacture and assembly of a gear train. Nobody repairs drills to care.

Once you loosen and remove the screw inside the chuck, you have to remove the chuck from the shaft. To do that, you may have to put an allen wrench in the chuck, tighten it down, and rap it with something heavy. Sort of like was suggested for the screw in the chuck. Except CCW iirc. (You can tell that by looking at the threads inside the replacement chuck.)