You can add a bog to your existing pond easily.  Select a location around your pond, preferably an area that is higher than your existing pond.  Dig up a small area the depth and shape you like and put down a piece of liner.  You can create a small waterfalls by placing a small pump in your pond and circulate the water from the pond up to the top of the bog area.  The hose was attached to the pump in the pond and then brought up the side of the bog area and buried in the dirt.  At the top of the bog the hose was clamped to create a small return of water into the bog and covered with rocks.  The edges of the liner were covered with rocks.