Attached Link: http://www.poserfashion.net/p4_to_p5_conversion.htm

You have to bring the conforming dress in as an obj before you can clothify it. What people do, often is export it as an obj and then import it back in and then it can be clothified. I don't know why the extra step is necessary but that's how it works for me. You'll find it doesn't conform anymore as a obj file so you have to the cloth room to get it to drape and pose over the figure.