Hi Steve,
The solution for your problem is the Substance Player.
It is a free tool which you can download here:
https://www.substance3d.com/substance-player/Once you've downloaded the tool, you will be able to import the .sbsars from your choice that you've downloaded from Source.
You can export the different maps directly to .jpegs, but you can customise them before if you want to:

1) & 2) In most of the materials (except some older ones) presets have been created. Those are ready-made customizations of the materials, and can help modify the material to a different look quickly and in a coherent way.
3) In materials, especially procedurally created materials, some parameters have been exposed. This allows you to change the color of the material, the size of the fabric mesh etc. so it looks exactly as you want it to. All parameters are custom to the material.
4) Once the material has the look that you want, you can export the material maps you need to import in Lumion, in the size you desire.
There is one additional very interesting possibility for the procedural materials: the random seed. It allows to have a new variation of the material but using the same parameters, allowing to get more diversity.
Let me know if things work for you or if you have additional questions!