Were there ever any extender pieces made for the SG Canals or RPK? If there are, then I' can't find them. If not, then they're the only canal/river sets I'm aware of that don't... Don't these two sets predate the whole concept of extender pieces? I wouldn't know how old they are...
I don't know since I don't use those canal pieces, but if there are none, they shouldn't be too hard to create. Just copy the building exemplar, generate a new Group/Instance number, then you can offset it using the SC4 model tweaker. You will then have to turn it into a prop and then lot it. It's a bit of work if you are unfamiliar with the modding tools, but it is certainly doable. :)