In article ,
"Bill" wrote:

I have a big hole where my tree stump used to be.

The lot upon which my home was built (1991) had many mature trees removed and
their stumps ground down to below surface level.

Is it as easy as adding topsoil, then some grass seed?

Yes.

However, you should be prepared for a "lifetime" of sinking ground and
topping-off with soil and re-seeding as the remnants of the tree rot away
below grade. My back yard is pocked with holes that can "never" be
permanently filled. It's an ongoing process. Good luck!
--

JR