I have a ranch home on Long Island that has hollow metal door frames as well. Wall construction is studs and drywall. I removed one of these doorways, and walled over the opening (it was a half bath with two entries, and I removed the one entry).
My door frames also serve as the trim around the door opening. The door frame I removed had three metal straps on each side that were nailed to the stud beneath the drywall. First, I cut through the layers of paint where the door frame meets the wall with a utility knife. I then used a reciprocating saw with a metal-cutting blade to cut through the metal frame in 2-3 spots on each side (about a foot from the bottom, a foot from the top, and just above or below the middle). I was then able to pull the pieces away enough to locate the straps. I then used the saw to cut the strap where it met the door frame. The pieces then came right off. If memory serves, the top of the door frame was not strapped to the wall framing. You can leave the straps in the wall. I was actually able to swivel them into the wall around the single nail that held them in place.
You will be left with a rough drywall edge, and studs and a header in the opening. This framing is what you will attach your pre-hung unit to. Don't worry too much about the rough drywall edge, as it will be covered with the new trim you install. If the metal also served as the trim, and you are installing a door the same size, then you may need to install trim that is wider than the decorative edge of the metal frame. The decorative edge on my metal frames is fairly narrow, so it would not be a problem in my house.