A quick secondary question I'm a bit confused on how I would go about combining several grey scale texture maps into one output > image. In unreal you could expose the RGBA channels in a node, plug in a selection of different masks and then spit those out into spec/roughness/AO for example Below I've selected the output…
Your substance can have as many outputs as you want. If you want a clearcoat texture, simply create an output node for it and it will appear in Unreal. As for packing images in a RGB, look for the RGBA merge node in the library, under Channels. This will allow you to combien maps into a single RGBA image.