RoughnessHelper.cs

using System.IO;
using UnityEditor;
using UnityEngine;

public class RoughnessHelper
{
	[MenuItem("Assets/Convert Roughness Texture %#r")]
	static void ConvertRoughnessTexture()
	{
		if ( Selection.objects.Length == 1 )
		{
			var texture = Selection.activeObject as Texture2D;
			ConvertRoughnessTexture( texture );
		}
		else if ( Selection.objects.Length > 1 )
		{
			for ( int i=0; i<Selection.objects.Length; i++ )
			{
				var texture = Selection.objects[i] as Texture2D;
				ConvertRoughnessTexture( texture );
			}
		}
	}

	static void ConvertRoughnessTexture( Texture2D texture )
	{
		if ( texture && texture.name.Contains( "Roughness" ) )
		{
			var path = AssetDatabase.GetAssetPath( texture.GetInstanceID() );
			var fileName = Path.GetFileName( path );
			var dirName = Path.GetDirectoryName( path );
			var metallicTexture = AssetDatabase.LoadAssetAtPath<Texture2D>( dirName + '\\' + fileName.Replace( "Roughness", "Metallic" ) );

			var output = new Texture2D( texture.width, texture.height, TextureFormat.ARGB32, false );

			var roughnessPixels = texture.GetPixels( 0 );
			var metallicPixels = metallicTexture ? metallicTexture.GetPixels() : null;
			var pixels = new Color[texture.width * texture.height];

			for ( int x = 0; x < texture.width; x++ )
			{
				for ( int y = 0; y < texture.height; y++ )
				{
					var index = x + y * texture.width;
					var smoothness = 1.0f - roughnessPixels[index].r;
					if ( metallicTexture )
					{
						var metallic = metallicPixels[index];
						pixels[index] = new Color( metallic.r, metallic.g, metallic.b, smoothness );
					}
					else
					{
						pixels[index] = new Color( 0, 0, 0, smoothness );
					}
				}
			}

			output.SetPixels( pixels );
			var png = output.EncodeToPNG();
			
			var destinationPath = new DirectoryInfo( Application.dataPath ).Parent.FullName + '\\' + Path.GetDirectoryName( path );
			var destinationFilename = fileName.Replace( "Roughness", "MetallicSmoothness" );

			File.WriteAllBytes( destinationPath + '\\' + destinationFilename, png );
		}
	}
}