1: add this line to the capability of whatever building you want to have it, where X is the number of bounus levels

population_health_bonus bonus X requires factions { barbarian, }

2: Sorry, I haven't tested it. I quoted it from someone else's research posted elsewhere on one of the big three forums, don't remember where. I think he opened the .exe with a hex editor and found a bunch of things romeshell might take as commands. Too bad it didn't work, I was looking forward tio using it myself.