How can I add a texture to existing materials…? Take any material from the Houdini material library, e.g. glass… I'd like to add a texture to control specularity or diffuse. Can't seem to figure it out or find and info in the docs.

The sample SHOPs like VEX Glass don't have this ability, however all the shaders in the Material Palette do. Just enter the VOP network and start tweaking.

To add a texture map to the Frosted Glass Material you'd need to add a texture map lookup to augment the Diff Color node inside the VOP network.

You can take a peek at the Basic Material VOP network to get an idea on how to do this.

Personally I think the first steps is to do some simple things in VOPs, and look at some of the tutorials. Best learn to walk before you attempt gymnastics.