How to insert an image into another images alpha channel?

I'm making a texture for a model that will be displayed in the source engine (specifically Team Fortress 2). Unfortunately the source engine doesn't like handling both a bumpmap and a specular map (environment mask) in 2 seperate files. You must place the specular map in the bumpmaps alpha channel.

I read somewhere online about the 'decompose' tool under the colours>components tab in gimp. When I do this, it does indeed break the image down into seperate channels in the RGBA spectrum (Red, Green, Blue, Alpha channel). So I paste the spec map into the alpha channel and hit 'recompose'. This is where the problem happens. The normal map has been made partly translucent and colours have dimmed.

TL;DR

Basically I want to insert an image into another images alpha channel, but not have the alpha channel make the base image look different, basically 2 image files stored into one, without having a visual effect on each other.