If you're really adamant about doing this, you can construct a reducer shim. Basically you'll need a block of metal split in half that is machined to fit into a 26.0mm opening on the outside for one end and has a 31.8mm opening on the inside. You will also need to drill properly placed holes to allow the cap bolts to fit through and use longer versions of those bolts. Note that this will push your bars out a little so you will want to slightly undersize your stem length. Syntace has this sort of shim system for mounting their 26.0mm compatible aerobars onto 31.8mm handlebars.
