I'm trying to figure out how to go about adding a chance of zinc to the crushing result of lead ore. Looking at the api I haven't noticed a clear way. Is this possible?

Uuuh. You mean to the automatically added one?
No, you can't modify recipes. What you need to do is add this via the API through, for example, this method, but BEFORE postInit() so that the recipe for zinc doesn't get automatically added :)