As long as the expander will grip the steerer interior adequately, it will work fine. I've used expanders in steel steerers as a temporary adjuater until I decided on the final trim height and they worked very well. I later replaced them with starnuts just as a cheaper alternative and to save an insignificant amount of weinght once I had the steerer cut to final length. It could have been used indefinitely.

As noted, you don't need an "official" starnut setter. A long bolt threaded into the starnut and driven with a hammer works fine. Once you get the nut started, remove the bolt, add the top cap and rethread the bolt. The top cap will act as a centering guide as you drive the nut to it's final depth.