KP,
as you are using oats and ale malt, I assume you are mashing and doing a boil. If so, I would say you are better off ditching the can of stout and replacing the majority of your ale malt with specialty malts such as crystal, chocolate, flaked barley and roast barley and get your extra fermentables with tins or bags of malt extract to put in the boil. This will give you a more flavoursome, "fresher" tasting stout. Look up some of the stout recipes in the recipe database to get an idea of quantities. The Oatmeal Stout thread in "flavours of the week" section is a also a good starting point. Obviously, if you don't use a kit, you will need some hops for bittering and maybe flavour - even better for a fresh taste!
Re your question about sweetness, it is generally dextrins from high mash temps or additions such as crystal malt which would make the brew sweeter. Adding more simple sugars such as brown sugar and molasses will make it dryer, not sweeter. They will however add extra flavour which can be good in a stout. Go easy on the molasses, though - it can be a dominant flavour.
Cheers - Snow