I'm wondering if I can
break up an existing concrete slab (the wife decided she wanted the steps
somewhere else) and use that as filler material --

I have seen "natives" break up concrete slabs just to use the pieces as
aggregate. No reason you can't do the same. If you combine the extra
aggregate with a bag mix, you might add some additional pure Portland cement
to the mix to keep the mix workable without reducing the strength. (The
strength is mostly a function of the water/cement ratio. If you add water
to keep the mix workable and don't add extra cement you end up with a weak
mix.)

If you don't want to break up the pieces to "aggregate" size, you can get
some mortar mix and put them back together as a "rubble wall."