You could add custom settings in hammer. Open up hammer with HL2 or something. go to 'tools'->'options'. Right at the top, you should see configuration, then a drop-down and an 'edit' button. Click the edit button, then add a new config for HL:S. Check the SourceSDK online docs for reference in filling out the config stuff, but you should only have to fill out "Game Configuration" and "Build Programs" tab. I don't know how textures are used (HL2 style or WAD style) and I also dont know if you would use the HL2 Compile programs or the older ones. I would assume the new ones because it is 'source'. Hope that helps.
Edit: Also, when you change the settings, close completely out of hammer and SourceSDK, then when you start it back up, the new config should show up in SourceSDK
