Well you could do it the old fashioned way (I've tried using the import manager too but it always came up short, and I'm skinning anyways and I find the old way much simpler.) Get
TechMPQ once you open it up just click on Add/Replace files, and in the first field browse and select the .blp file, in the second field select your map, and in the 3rd paste in Textures\HeroDemonHunter.blp and voila! Your skin will be there unless you save (which means you have to do this everytime after you save your map or even test it, so if you want to see the skin in your map, follow the above directions and then open the map in WC3
without saving our testing it in WE.