There were a good number of posts about this when LP8 was first out, lots of people installing and wanting to do this. From those posts I remember some people advising that you need to dump the Apple Loops Index files. They can be found in Library->Audio->Apple Loops Index, Your User->Library->Audio->Apple Loops Index, and Library->Application Support->Garageband->Apple Loops Index.
If I were you I'd try what Joe77 said WITHOUT doing what I stated above, and then if that doesn't work, try to dump those index files.
I also recall from seeing a Logic Pro 8 video that when you drag them into the loop browser as Joe77 explained, (if you've done it correctly) it will as if you want to move the loops or keep them in the current location. Of course select current location.
As I said, there are a lot of posts on this topic. But I don't recall seeing one from someone who was successful in doing it, with a step by step for what they did. When you get it figured out, please post a follow up here with what you've done, as that will help out other forum users with the same question.
Good Luck!