Can I use an expanding plug rather than a star fangled nut in a steel steerer?
Can I use an expanding plug instead of the star fangled nut?
Yes you can.

I thought I would expand this answer to say that:
-certain expanders add an extra 1-2mm to the top of the steerer tube, meaning you may need an extra spacer depending how you cut
-Quality steel steerers are thin walled so you may need to shim your expander with e.g. some inner tube if you need a bit more bite. Apart from eliminating a sharp edge and making things look neat, they only hold the bearing preload and don't hold the assembly together. You already know this but I've seen enough bikes with Mega Torque on the preload bolt in the last 3 months to know that there are a significant number of people who don't know how this system works.