To get the original material back, right click on the material and choose Reconvert from Poser.
You can also reconvert all the materials of a prop, in this case right click the prop and choose Reconvert material(s) for this figure/prop
The LiveDB materials can be tweaked as well, but many of them are a bit more complex.
There are two special nodes you have to understand: Material Mix and Mix.
Mix is simple, it is like a blender node in Poser - it will split a texture or color in 2 textures or colors and a mask or value. With this you can mix 2 textures depending on a mask or value. Example - you have a mask (black spot on a white background), Now with the mix you can set the white part to blue and the black part to red with the mix node. In your screen shot the mix is used to vary the specular value.
Material mix is essentially the same except it does not work on a single node but on the entire material. It also uses a mask or value and 2 materials. With it you can for example turn one part into glass and the other into wood.
There are many node types you can use, each with different properties. Easiest thing to do is to load a box and try out the different materials in LiveDB on the box to see what they do and play with the properties of the material. Most of them are relatively simple and are of the material mix type. A simple way to get shininess on a surface is to use the original material in the first node, a specular material in the second node and set the value anywhere between 0 and 1. Changing this value will add more or less shininess to the material.
Configuration: Windows 10 64bit Creators Edition, I7 6850K, 128GB, 2 GeForce 1080ti, P11 Pro SR6