How can you download custom hair from sites into Sims 3?

Is there a way to download hair made by people online into Sims 3? Like you could in 2?

I don't remember how you did it for 2 but I know it wasn't super difficult.

So I know you can download hair to your computer, but how do you actually get it into the game? I found a few that I really love and want to use.

And I don't mean the ones that you buy off the actual Sims site haha.

Thank you!