It's not completely straightforward but with a bit of digging into the nodegroups of the materials, adding a new material is easy. Just duplicate a material that is sort of similar, then duplicate the NodeGroup also and rename both to the new material name. Then change the textures inside the Nodegroup and you're all set.
Yes, it does. Both Cycles and Eevee. The showcase projects are also prepared to work well with Eevee.