exterior door - sheetblock?

I am in the process of converting a steel building into a studio. So far, so good. The exterior doors are steel - and hollow. The plan is to keep these doors for now and add another door inside. The walls are about 14" from the outside to the inside. I saw the thread (with pictures) regarding the exterior door using sheet lead. I have some sheetblock type material (sonex's version) left from previous projects. Will that work instead of the lead? Would it be beneficial to use more than one layer? I have plenty. Thanks! :) Scott