I don't think I've ever seen a WW boat where someone converted from saddle to seat.  Not sure how low you would have to hang the seat to maintain at least moderate stabilility while sitting.  Maybe you could get an idea of how low the seat would be from pictures of those hanges and put something in your boat to simulate that height and see if it will work before you buy one - just sitting in a lake will give you a good idea if that height will work.  One thing you can try is just putting some minicell foam on top of the saddle you have and see if just raising that up a few inches helps you out - just tape something over the saddle for a test - even if it does not solve the not being able to sit comfortably issue, it will give you a feel for how much stability you might lose by sitting x inches higher - you can gradually build up the height untill it is close to where you would have the new seat hung, and see how it feels.

I occasionally sit in my WW boat if on a quiet section of river, by perching atop the back of the saddle - it is quite tenuous, and I only sit that way for a few minutes to stretch my legs.  Without looking at the boat, I'd guess I was sitting at a height that is maybe 4" below the gunnels.  I wouldn't want to run thru anything more than a riffle that way.

I'd really suggest that rather than changeover a WW boat, you just keep an eye open for a decent river tripper on Craigslist.  For many of the trips I do, even old Coleman, Grumman, OT DIsco,  will work just fine, and you can pick one up for maybe $350 sometimes - I have seen several better  better river trippers posted for not much more.  

That way, you still have a boat capable of more serious WW, and a boat for more moderate stuff.   The right tool for the job is a lot better than trying to make do.