All HL game systems come with an editor tool that makes adding new "Things" possible. These new things are stored in files called .user files which are actually XML based. You will have to become familiar with the scripting language HL uses. In this way you will find more support for other game systems like Pathfinder that have had LOTS of things added to it over the years. Check my signature for help places and info. Allot the basic building blocks like "Things" and "Picks" are HL terms and not specific to a game system.
The above info is also supported by what Liz says
HERE in her last statement of that post.