We have some variations on how to retrieve roughness, metallness and AO values of a texture (see useRoughnessFromMetallicTextureAlpha, useRoughnessFromMetallicTextureGreen, useMetallnessFromMetallicTextureBlue, useAmbientOcclusionFromMetallicTextureRed) but it would be too complicated to support all possible combinations (like having a different texture for each value, or one texture for roughness+metallness and another for AO, and so on).
If your assets are not designed this way, you should preprocess them to avoid having to rebuild them at runtime, although this could be a nice feature to have, at least for a quick demo / prototyping (but using GPU-compressed textures would not work, as you cannot recompress them on the client side). @Pryme8 is making a tool that let’s you create a texture from different source textures: Image Channel Swizzler