The levels in the PS2 version of Half-Life aren't extended per se, but in certain areas they have been improved significantly over the PC version. That means in some cases there's an extra room here and there (Before the Hazard Course you actually get to walk around room that the elevator brings you from, there's a small added transition room added when you're swimming through a pipe in the Lambda Core chapter, etc). I don't know if these would qualify as "extended", but the PS2 version of the HL levels do contain improvements and small additions.