You can vent a free-standing wood stove through a masonry chimney without a stack pipe inside (sleeve), but the problem with an insert, is when you clean the chimney, it just falls on top of the stove. Your best bet is to use 6 inch single wall stainless stove pipe topped with a spark arrestor and rain cap. This will stay hotter, cleaner and will prevent the need to remove the stove when you clean the chimney. Check code and professional installer recommendations at a local stove shop.