I'm personally thinking this will not be a good idea/work. Putting a pump on the outflow in my opinion would actually add stress to the canister because I would think the canisters pump would be fighting to keep up with the additional output pump you have added. Like in my mind I see the second pump sucking the water out of the canister faster and damaging the actual canister pump.. then on the other hand putting one on the intake side I only see problems as well as you would really be sucking the water out of the aquarium faster and then the canister pump not working with that as well.
These make sense in my mind and may be completely wrong but I just see the two pumps "fighting" with each other and the whole thing not working out... plus I personally wouldn't risk wrecking a nice canister.
Now if you really want to add some more inline peripherals I would just run a pump that is hooked up to just those peripherals. Sure you will have more tubes in your tank but I see that as a more feasible way of things working out. This is one of the reasons my new tank is going with a sump setup. Everything will be out of the way and it is quite easy to change things if need be!
Well good luck with whatever you do and if you do try something let us know because we would love to hear and learn!