You would need to change the model of the buy in CS to the scientist, which is very possible, seeing that they did it in Italy. This will not work in HL though, although there is a way you can mimick it.
Make it so that when the scientists is in a certain point, it triggers whatever you want to happen, like a sound or a door opening. Scientists will follow you if you press e by them.
Also, you can have it so that when they die, a black screen pops up and says that you lost.
If you have questions about how to do this specifically, just ask.