Sigh, to have a crush on someone who is already taken...
There's really nothing you can do. I wouldn't want you to be the third hand and break up a perfectly good relationship.
I would say that there's nothing wrong in trying to befriend the person though. It never hurts to be their friend.
If you can't stop thinking about him.. then don't stop thinking about him

hehe j/k j/k. Try to indulge yourself with other activities such as drawing, reading, etc.. to get your mind side-tracked off the person.