The core system for Werewolf, Vampire, Mage, etc is the Story Teller System. So if you have one of those games then they probably use that system.
As per Shadow World, I have to admit my ignorance as I do not have very many of the books (I think I have 1 or 2) so I do not know a whole bunch about it, just the info I have received based on other GM's using some adventuresses for their home games or letting me borrow and read what they had way back in the mid 90's.
But if there are multiple moon's then you could do a whole bunch of things as Terry said above, different tribes could be forced to change during different moons, different breeds could be forced to change during different moon or you could do something completely different just for your home game.
I do not remember a lot about the essence flows, earth nodes and storms but I think I would tie the were's and other creatures to them in some way shape or form. In the WW product were's already need lay-lines and earth nodes to some extent and compete with Mages and Vamp's (to a lesser extent) for them. Were's are also tied to some extent as guardians of the land and that aspect could be exploited as maybe essence storms or earth nodes create such a group or groups.
Also IIRC there were some very powerful spell lists in RM2's Spell Users Companion for genetic manipulation and maybe someone could use those on a grand scale to create many such creatures.
The one thing I like about how WW tackles the were's is that they try and force some humanity on "them" and not just portray them as mindless killing machines, the same can be said for Vampires.
MDC