Adding new surface properties

Sorry if this is the wrong forum and this should go in "Art" instead, I
just figured that since it was technically an editing question...

Anyway, I recently started messing around with new materials and have
created a new snow texture for use with HL2. In order to get the proper
sounds etc. I have to define a $surfaceprop value in the vmt file.
However, HL2 does not have all the surfaceprop values that CS Source
does (e.g. basic snow) so I need to create a new one. A brief bit of
Googling has been no use on the matter. Any ideas where to find the
file that defines them, or how to add a new one?

I think mappers have to define the properties for each individual material in each VMT.

Like setting the impact sound and slippery-ness.

Are you sure about this? The standard valve materials tend to have
$surfaceprop followed by something like "metal" or "dirt", and possibly
a few proxies, but that's it.