pullovers def. "expand" your ribcage. i've def. noticed thickening around my whole chest/back (hence the ribcage) by doing them consistently for a few years now.
when doing them.........
-keep your ass down as far as possible and don't raise it at all
-lower the weight as low as possible. when doing so, take in the deepest possible breathe you can (which i believe expands the ribcage).
try doing that exercise adn you'll notice great results in rib expansion as well as bringing out your serratus's.