Almost impossible, I'd say (as a previous cat owner with a veggie patch). They always go straight for the best patch of nicely dug earth, so perhaps if you can prepare a little patch of losely dug earth near your kitchen door, your cats might be lazy enough to poop there. But knowing cats, don't bank on it ! My cat often pooped on my veggie patch and I'm here 20 years on without having been poisoned, so maybe you have to be a little pragmatic about it, or buy a pooper scooper and do a daily collection. The alternative is to have lots of regularly changed litter trays indoors, and that isn't always pleasant either.