It has to be designed with a single solid group for the part you want to make dynamic, and most dresses don't do that. This is why I've mentioned this dress was designed especially to allow this functionality. I think it is only possible with weight mapped dresses and skirts. I think legacy Poser/DS grouping requires split groups for the legs and hip, and this will not work for semi-conforming. My other dress for Dawn, the "laced sundress" was also especially designed for semi-conforming, and it works well with a that because the grouping was meant for it.
Check if your dress has that kind of grouping at the bottom. If not, you would have to redo the grouping and rig it again. The rigging can be made easier if you transfer the weight from the old to the new version, but differences in vertex count and order will introduce some spikes that need to be smoothed out. An easier way would be to convert the dress figure into a prop and run the whole thing as dynamic. That would be quicker, but you have to make sure all groups are welded. You can do that in Poser with the Grouping Tool.
Well that's one way to put it. 
