Orbit Darkes

I have some 3D models that are textured using a PBR metallic/roughness workflow that I want to use in SL. Reading these forums it seems there will be a lengthy period of transition before the majority start using PBR enabled viewers, so I would also like to texture the models using the legacy Blinn-Phong approach, of which I know little. Is there a way to manually edit my existing metallic/roughness PBR texture maps to work with Blinn-Phong? I’m unable to test myself right now but I am wondering if this would work: Blend the AO map with the Albedo to create a diffuse. Use the normal map as is. Invert the roughness map to create the specular. This is the bit I'm most dubious about. Would this work? Is there a better process to use? Thanks a lot!