Yes you can. You can unpack the assets and use them in your Project. You will need to create a new .rez file when distributing the assets because you can't rely on everyone owning the game to have the expansion pack installed.
It's been about 10 years since I've done it. Someone else will have to explain how. Off the top of my head, I think WinRez is the best tool for unpacking assets. Unlike LithRez, it has Windows style UI which makes it easier. That is, if I remember right. lol.
-Windebieste.