Since you already have laid the base and the purpose of the extra width is there to prevent all the weight from being right on the edge thus prevent it crumbling. I would dig out a trench all round the base to the depth of the keels allowing an extra 150mm all round to be added. This could be reinforced with rebar and as a belt and braces to tie the extra concrete to the existing base I would drive some long concrete screws into the side of the existing base with about half the screw protruding.

Richard