tall order. Designing clothes in either one would call for modeling the objects. I'm going to be burnt at the stake for this, but what both Daz and Poser are, are actually a way of collecting various bits (hair, clothes, textures, figures, props, etc) into a composite picture. While it is possible to model clothing in Poser, it's easier to do it in a modeling program, then bring it into either to allow it to be rigged, boned, etc., to fit on the models.

You could start with Wings, Blender, Amapi (all free ) for starters, and then if you like the idea, you can go on to the 'spendy' ones like Silo, Hexagon, etc., and if the bug bites hard, Lightwave, Maya, or Modo.

That being said, I'm not up on transgendered fashion, so you might take an alternate route, and take existing clothes for Poser or Daz, then use morphs and textures to alter them. Probably cheaper, not sure if it's easier, tho.

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)