Making semi form-fitting clothing and drapery is well handled by the new topology functions in Z. Go here…
http://www.zbrush.info/docs/index.php/User_Created_Topology_and_Reflow_Lab
… and read through the topology lab, then create a skirt mesh that hugs the contours of the legs, but is also bridges the gap in the middle. Draw the new lines out with their eventual drapery in mind…for example, droop the lines in the center into that “smiley” shape. Extract the new geometry as a relatively low poly mesh, and add it to your body mesh as a subtool, or a mesh insertion. In the lowest suBD level, pull (“move” brush) the verts along the bottom of the skirt to occupy the general landmarks you want the skirt to flow to, then sculpt the drapery and transitions on higher poly levels.
Keeping the skirt mesh low poly to start is important, as its much easier to pull the verts to the general landmarks you wish. Your image shows a mesh that’s a bit higher poly than it would be easy to establish the basic form for flowing fabric that might fold back on itself at points. Subdivide as necessary to smooth it out and sculpt the fabric.