Clear Texture's Alpha Channel?

I’ve got a couple of textures-sets that have maps for diffuse, normal and specular, but not for roughness. I’d like to use “Texture Compositing” to use the normal map’s own alpha channel with the “Add normal roughness to alpha” option. So then I would have RGB for normals and A for roughness in the same texture. The problem is: the alpha channel is already full white and I can’t add to it :confused:

So, I need a black/zeroed alpha channel. There doesn’t seem to be a way to achieve this in UE4. I’m not much of an artist and the only picture editing programs I have installed are windows paint, which seems to completely ignore the existence of alpha channels, and GIMP. I tried making the texture all transparent, but this affects the color channels as well, making them useless.

How can I solve this?..

first of all, specular is as close as useless in ue4 as possible, so you could replace that for your roughness map.
second of all, the normal map compression actually uses the alpha channel as well, hence why you cant use it.

you could get rid of the specular altogether and put the roughness in the alpha of the albedo (note!: not diffuse, as ue4 is a PBR (physically based renderer) diffuse is outdated, and you will need an albedo texture… which is basically the same as a texture but without ANY shadow detail/info in it)