Can soap be put it in the water used for Sous Vide?
I'd be concerned with two things here.

First of all, I'd be concerned that some soap might contaminate the food product without you knowing about it.  If it's a low enough concentration of soap that it would not cause significant harm to ingest, this isn't a big concern; on the other hand, I might be inclined to use something other than actual soap that is able to accomplish the purpose here if that is possible - for example, a mild bleach solution (I've also seen "pool shock" suggested) might be safer.  Do some research on whatever you add to make sure it's actually okay for this purpose.  If your purpose is to remove bacteria, this is probably your best bet.

Second, I'd be concerned with odors from the soap affecting the food being cooked.  Even if the soap never contacts the food, the odors that make the soap smell good are much more likely to affect things in my experience. If you do use soap, use an odorless soap (something like original (blue) Dawn, no endoresement suggested).

If you can edit the question to provide more of an explanation as to why you're asking it (you mention Kosher laws, but not the specific issue you're trying to address) you may get a better answer.