You can accomplish the same thing with a clean spray bottle. Spray it into the intake while the car is running....in bursts that almost kill the idle. Or rev it and spray more at a time.
It'* best to disconnect the downpipe first. You may dislodge chunks that you don't want in your Cat. A fogger system of water injection won't dislodge huge chunks like that. Little bits at a time that will pass through the cat and muffler.
In the long run, it'* probably best to set this up BEFORE finishing your exhuast