I know that you can make a reactor out of PVC piping and fittings(eg. search rex grigg reactor). But in my opinion, it is a waste to use a canister filter just as a reactor.
The idea of a reactor is create more time for co2 to be dissolve before leaving a container. Therefore, answer to your question is that you can try just feeding co2 into the inlet of your canister. If you don't see any visible bubble from the outflow, then you have yourself a reactor.
Forgot to mention: If you feed the co2 straight to your canister, your canister will have a much shorter life.