Thinking here nova.............lol
The main thing you need is a non leaking connection between the two. If you cut down the side, where you want your water fall you could put in a piece of liner, with a lot of the pl roofing sealant or some kind of sealant secure the liner to the inside of the bog, then have the liner go into the pond.
You could, with cutting the edge, folding it down a little, put the liner below the lip, secure it well with sealant. Use sealant to make the water path correctly out of the bog. I use sealant with some rocks when I am attempting to path the water the way I want it to go. Make sure your liner is cupped a little center lower than edges so the water flows down it not over the edges.
If you have any liner tape that would also help you attach the liner to the bog. If you secure the liner on the inside of the bog you want the edge to have no way for the water to flow under it, i.e. well sealed, smooth edge in the bog
Hope that makes sense.