Ok, you said you know how to extract the skins, correct?
When you extract the skins from the MPQ (assuming you're using Warcraft III Viewer), go to the Models tab of that same menu. Find the model that matches the skin you just exported, and click on it.
Click CTRL+M for the textures to pop up.
There should be a texture or few textures listed.
You want to find the path of the texture that has the same name as the skin you exported.
Once you have that path, import the skin into the World Editor. Double click on that texture and check "Custom Path" box at the bottom. Paste in the new path, click ok, and it should be good to go!
Ask if you have any specific questions on a certain skin because that would be easier to detail instructions : )
Good luck,
electricsaiyan