I would love to see a PBR setup in OpenMW, it would certainly get me a lot more interested in modding Morrowind again. It is basically the standard these days, even Blender defaults to a PBR workflow for both Eevee and Cycles now. It isn't that hard to make maps for it manually. Metalness is nearly always a binary thing, pixels should be either black or white. Black when they aren't metal, white when they are metal. It's best to think of it as a switch that's either on or off. Stuff like rust and dirt are not metal, and should be black. You can have a little variation in the near white end of the spectrum, it isn't strictly accurate, but it some times look good to lower the value a bit for older metals, but you want to keep metal stuff above 0.8 (0 is black, 1.0 is white). You will have to make it a point to look up the base color value for the given metal you are working with to use with the albedo, this is probably the least intuitive aspect of all of this for newcomers, but luckily there are really good color charts out there for PBR metal colors.
Albedo is the replacement for diffuse, it is basically just pure material color without any baked in lighting. So the general way to convert existing diffuse maps is to level them out, using like the levels or curve color adjustment tools in your preferred image editor of choice. You just want to get rid of the really dark and light areas of baked in shadows/lights. But you don't have to go crazy with this, most game engines end up allowing to mix in an AO map, which again isn't strictly accurate, but often looks quite good. Just keep in mind the above point about metal color.
Roughness is pretty simple to generate. You can look up the roughness value for a given physical material. A mirror would have a near 0 value, a dry dirty coarse stone would be close to 1.0. So the way I go about it is to take the albedo, convert to grayscale (you may also want to invert it, depending on if you want the lighter or darker areas of the albedo to be shinier). Then open up the levels tool, I leave the top part alone and focus exclusively on the bottom output range (in GIMP). And say I find out the material I am working on says it should be around 0.6 roughness, I would set the lower bounds of the output to say 0.55 or 0.6 and the top end to something like 0.7 or 0.8. That will compress the gray scale values to a range that is a bit below and above the target. This can take some testing to find an ideal output range for a given material, but its fairly reliable and gives pretty good results.
Anything beyond those is mostly optional, and most other supported channels can often be handled with flat values. But a subsurface scattering (SSS) map can be useful for flesh and such. If you want to support some sort of glowing effect, usually emission maps are colored so that black is no emission and the color used is the color of the glow. This can also be done with a simple black and white mask, so black areas again don't glow and white areas use the color of the albedo to decide the glow color. For that to work you would basically mix the albedo and the mask and feed the result into the emission.
Anyhow, I'll wrap this up by saying that PBR support would make the post 1.0 OpenMW a lot more attractive for those considering using the engine for more extensive modding projects, like graphical overhauls, or potentially for all new games.