Hi, you won't have much trouble if they are uv-eed. If they are not you will need to unwrap them in a uv program or use procedural textures (don't know much about poser materials sorry)

If you go to their textures and open them in photoshop and they look like they make sense - eg flags or porthioles or doorways etc, then they probably have uv  coords.

Just save the original textures under another name and make your own textures over the top of the old one.

If you can't figure out what texture goes where or the orientation just put different colours in parts of the texture map, then reload the textures on your 3d model and it will give you a clue.

Just remember save the originals first :) That will save you having to change texture maps in the materials room

show us the resuklts when you are finished Ron Deviney has great texture brushes for grunge and Old Technical stuff (at daz site)