Disclaimer:  I'm confident that my way is by no means the best way to do it and am certain that there is a better way to do it.  That being said...

(Assuming you've got all of your textures saved as .jpgs or .psd, etc...)  Also, assuming that you're using Windows as opposed to Mac.

First create a folder in Textures under the runtime directory.  Example c:program filescurious labsposer 5runtimetexturesPropschick.

The way I've done it (and I don't do it very often) is to go to the Pose directory in the runtimelibraries folder and pick out any ol' existing .pz2 file which is associated with applying textures (materials.)  Right-click on it and choose Open With....

Select either Word Pad or Note Pad to open the file.  Start scrolling down and find all of the instances where you would see something like the following:

textureMap ":Runtime:textures:Stegy:BaseTexture_Body.jpg"

Replace it with your own file information, perhaps something like this:

textureMap ":Runtime:textures:PropsChick:BaseTexture_Body.jpg"

After you've done all of your replacements, do a Save As to a new file name.  The only thing left to do will be to create .png thumbnail to go along with it. You can do that by rendering a scene with your new texture in a 91x91 window and saving it as a .png picture.  Just make sure that it has the same name as your newly created .pz2 file.  That's it in a very brief nutshell.

Edit:  optional stuff: You can also create a .rsr file (alternate thumbnail file format).  Just go to the Freestuff and do a search for "RSR".  Look for a utility called, "PNG TO RSR"