They've been supported as textures since Poser 4 at  least, don't know about 3. I believe that some apps like C4D will allow you to load PSD layers into different material channels, though as pjz99 says I doubt that Poser has the ability natively. BB might be able to find a way to do it but ach the file sizes. If I wanted to view changes while in Poser, I'd probably just export the revisions to .jpg.

Something interesting is the "layered image editor" plugin for DS which will apparently let you use .jpg files and create layers for things like tattoos and then apply them with a MAT file. Sounds a lot simpler than doing it with nodes.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken