You could look at the docs for the Voro++ program (which compute voronoi/atom uses)
to see if it gives you the info you need. If it does, there might be a way
to add an option to compute voronoi/atom to extract more info from Voro++.
Since the compute returns per-atom values, you'd need to think about how to
calculate and return something like "coords of an open-space point near this atom" for each atom.
If Voro++ doesn't do something like that, you could talk to the author, or you
could figure out how to compute it yourself from info Voro++ returns. You might
be able to write a compute that invokes compute voronoi/atom, then uses the result
to compute what you need and returns. Then use that result as input to
fix deposit or create_atom single to add a particle at the calculated coord.