The Parched

Is it possible to feed up a parched and turn them back into a person?

Possibly. It would likely depend on how far gone they were, for how long, how much you fed them, and if you happened to have someone more knowledgeable or able to perform the requisite 'dark miracles of science' on hand if you found it wasn't working the normal way.... I'm sure one could be found easily enough.. for a small, entirely irrelevant fee of course.

A haemonculos could.

But then, haemonculi can do anythng if they torture enough people. When all you have is a well-equipped torture chamber...

You could consider the point that if they can resurrect the long dead (whose souls left heir body upon death) , then it should be easier to top up the soul of someone who's been mostly drained, I think the only reason we hear about it is because any kabalite worth his salt should be able to keep himself full

Theoretically absolutely and they have the added bonus of being a cheap labour force, as I understand it parched are like heroine addicts, they do anything for their fix. The question would really be how much would you want to 'fix' them? After all they got themselves into that state.