"an Extrude modifier and then a cap holes modifier" Good, this works also. The other solution was probably turning invisible because of welding problems.
Thank Peanut! The way I got around the problem was adding an Extrude modifier and then a cap holes modifier. Somehow that made it work, oddly enough. If I had only used Extrude or had chose to convert it to editable poly, it would just turn invisible. For some reason adding both extrude and cap polys made it extrude…