How about editing (transparency to translucent) an already made OBJ, with no MTL file to start with?

There's a "cheat" to create a functional MTL file for an existing OBJ file. You can edit both OBJ and MTL files with a simple text editor.

Example for test.obj:

In the OBJ file add a line directly after the top description lines (#) in the OBJ file (if there are any):
In the OBJ file there are lines starting with "g <a name>". After each "g <a name>" line you can add a line with For example: Each different name you use will become a color/texture property name. You can use a different name for each g or use the same if you want multiple "g's" to have the same texture.

Now create an empty mtl file: test.mtl.

For each <your name> you added in the OBJ file you add a line Save the file.

Now import the OBJ file into Sweethome3D. Obviously there are no colors or textures yet but you should see the property names you created when you modify the imported object. You can set your colors and textures and when you export the object a new, complete MTL file will be created by Sweethome3D. If you want to add transparence to a specific color/texture just add the line with "d" to that property after Sweethome3D created a correct MTL file and import your object again.

It sounds complex but it's actually very easy. Good luck!