Some barrels will have a thread for attachments on the outside, sometimes it's on the inside of the barrel. Put your finger into the barrel muzzle to have a feel for internal threads.
In future there's a "quick questions and answers" thread which might be better suited for this, it's stickied.
Good luck!
P.S. If the barrel doesn't have any threads I think you could probably get it threaded, you'd need to know the thread dimensions and find someone with a suitable die to cut it.
