Two sugars available for building body in sour beers are maltodextrin and lactose. Lactose will have a bit more sweetness than maltodextrin when added back to a sour beer so be forewarned. Both can be a bit of a pain boiling up to add back to a thin beer, with maltodextrin being more of a PITA (easily boils over). As brett and bacteria can consume both of these products, albeit fairly slowly, you need to be able to keg your sour beer and keep it cold so the microbes don't continue to consume the sugars and thin out the beer again.
As much as this procedure will work, I would only do this if you are at your last resort (and you keg). Blending beers (a non sour or less sour with your very thin sour) is a much more traditional method and preferred.