I suspect it is a body part with a non-standard name.

Do this.... Convert with the As Prop box selected and the Regroup box unselected. Does the skirt part remain?

If OK apply the figure file from the first skirt. Do this by taking the new skirt prop into the set up room and double click on the first converted skirt's library thumbnail. You'll then need to add a bone and name it to match the extra skirt group name.

That will probably need further explanation but try that and see how far you get.