If you are wanting to use Poser models there's a professional clothing design program that outputs actual dynamic clothing for Poser models called Virtual Fashion that can be found here:

http://www.virtual-fashion.com/html/en/index.php

But if you're just interested in the designs, you don't even need Poser because you can model the clothing right inside VFPro itself. I've used the Basic version for all of my Freebie clothing and the Pro version offers the ability to export to more Poser figures.

Since you appear to be more interested in the actual fashion design aspects and not so much needing Poser, i highly reccommend it as a stand alone design program. Check out some of the free Poser clothes I've done here:

http://www.renderosity.com/mod/freestuff/index.php?user_id=454914

My Freebies