Unfortunately that kind of variable setting is mostly not available.
In ExpandedFactions I have a submod that increases health regeneration in cities to Citylevel * 5. This is a mod that does the same, but only for un-modded cities.
http://pastebin.com/raw.php?i=EjkgvVBu (all cities have 5 by default, so it is not necessary to mod level 1, and I subtract 5 from each other citylevel (level 5*5 -5)
