When pouring concrete, is it safe to add large rocks to increase volume?
Concrete is a mix of large aggregate, small aggregate, and cement (a 4:2:1 ratio is a good approximation - though designed mixes will be more calculated than that).

The size of the large aggregate isn't particularly important, unless you are working in very tight spaces or around reinforcement, in which case you want suitably small aggregate.

In this case there won't be a problem at all using softball-sized rocks. Just make sure that they are clean (ie don't have soil stuck to them).

(I remember seeing a section cut through a piece of concrete at the Hoover Dam Visitor Centre, and some of the large aggregate in that was rocks up to perhaps about 8 inch / 200mm or more).