You find it on my github and it is for free:
https://github.com/jayanam/fast-sculpt

You can use it to extract meshes from masks and union the meshes you sculpted on with the target object.

In this video I also explain the python code for the addon so that you can see how to implement an addon like this on your own.

The addon works with the new Blender 2.81 experimental build because it uses the new voxel remesher from Pablo Dobarro, you can download it here:
https://builder.blender.org/download/

See my social profiles here
G+: https://plus.google.com/+JayAnAm
Twitter: https://twitter.com/jayanamgames
Facebook: https://www.facebook.com/jayanamgames
Patreon: https://www.patreon.com/jayanam