Just feed it fattier foods, we feed ours All Stock grain and they seem to do great on that, i knew some people that fed theirs almost only bread with water on it to make it easier for the pigs to eat. they had the fattest pigs ever! I'd rather have meat than fat though, but thats just me.
whole milk, raw cow or goats milk? or you could ever try milk replacer powder/mix from the feed store, our pigs sucked that up like nobodies business when we mixed our leftover calf milk replacer for them. maybe mix it thick to make sure they get most of it..?
ETA: our pigs main diet is all stock grain and lots of produce(we get it for free from the grocery store at the end of the week), plus kitchen scraps.
ours stay pretty lean, but still heavy & meaty, just not fatty.
I'm trying to remember what I fed mine when they were underweight before the fair years ago....but the name is illuding me...It's basically a milk replacer . It comes in a 50# bag , it's powdered...so you have to add water . Mix it with grain or feed . Mine gained enough weight in a week to be able to show . Try calling your local feed store . Good luck !