Yes, the texture decal is blended with the diffuse/albedo channel at this point.
I’m not sure that being able to choose the channel is enough, as you’ll probably want one part of the decal to be colored (perhaps the outer part of the bullet hole) and the other to be transparent (the inner part of the bullet hole). So you would need to be able to provide several textures for a decal (albedo, opacity, bump, …). This is not planned for the moment, but PR are always welcome 
In your case, you can use a material plugin as a workaround: