No, modding support was disabled after the whole hot coffee "mod". For some bizarre reason the steam version also doesn't support widescreen resolutions. The original retail version did. Both can be enabled again by:
1. Downloading a NOCD crack for version 1.0.
2. Overwrite your gta_sa steam version with it.
3. Have fun with mods and widescreen support.
Why doesn't the steam pack include GTA London 1969 and 1961? I know that they did poorly at retail, but GTA1 and 2 are free. So why not include both of them