I only found an option to “add normal roughness to alpha channel” from another normal texture. But that’s not really what I need.
I am putting together some landscape textures and (since samplers are limited in number) I want to use the alpha channels of diffuse and normal map for roughness and specularity. Since these are both grayscale, it seems like the best solution. But can I “compose” textures like this inUE4 or do I have to do it externally?
Most likely you will have to do it externally.
Be advised, however, that storing alpha channel virtually doubles memory usage of your texture, and using additional channels in normal maps provides a bunch of complications.
There are some discussions on the forum about that:
Hmm, I wasn’t planning on investing too much time on this 
Is there something you would recommend instead?
Like, not using alpha-channels at all but rather a third RGB-texture that stores up to 3 extra grayscale maps?
I WOULD require fewer samplers by using alpha channels, though, right? So it’s “sampler-eficiency” and possible glitches vs memory-effeciency?..
Storing 3 masks in RGB texture is a good option, if you don’t mind that one mask might slightly bleed into the other one.
If samplers are your ultimate limiting thing, then you do not have much choice, but to use those extra channels.