If the hogs have a parasite they need to be treated individually by someone who knows what they’re doing. Adding something to water would be too random i.e. there would be no guarantee which hog/s were dosed, how much, etc. Although how do you know they have a parasite problem – did you have the poo tested?
But, if necessary, it may be possible for them to be treated without having to stay. I would ring your local carer/rehabilitator, if I were you and discuss it with them.
I wouldn’t just stop feeding them, though, they may be relying completely, at the moment, on the food you’ve been providing.
Good luck. I hope all goes well for them.