Here's a dusty thread that talks about a slip-on / set-screw adapter that will give you standard threads to which you can attach a flash hider or other barrel-end accessory.
I think most owners of the non-threaded 15-22's have opted to simply have their barrels threaded. There may be some place local to you that can help, or there are some folks in this forum that are top-shelf artists in the trade that might be able to accommodate your needs for a modest fee. Either way, best of luck and welcome to the forum!