I don’t know a skin editor that offers semi-transparency other than the full eraser tool, BUT there IS a website I use: photopea.
It has everything that photoshop has, and it’s on browser so you don’t need to download or pay anything. If you know how to use layers and opacity, you can make transparent and semi-transparent PNGs!
Something like a tablet would work the app is on google play store here is link: https://play.google.com/store/apps/details?id=com.wandapps.multilayerphoto basically you add a skin file first the add a photo over it which will be what you want to combine it with.