The problem with softwood steps is that a good polyurethane finish will be harder than the wood. If enough weight is applied to the step, the finish will crack. It can work, but regular maintenance will be required. You could create "hardwood steps" by covering the existing steps with 6 mm (1/4 in.) thick prefinished hardwood floor panels. They can be glued into place over the existing steps. Since every step would be covered, the height of the steps will remain even. The only problem with this project is that you will have to install wood trims on the edge of each step. If you add another layer of wood to the existing steps then two layers of wood will be visible at the step edge. Find a wood trim that matches the new "hardwood" steps to cover the edges.