You will create particles to load into Katana in some other piece of software, such as Maya or Houdini. You will export them and bring them in to Katana via Alembic or some other mechanism. Be sure that when you export particles for use in Katana that the per-particle width primitive variable is also exported. Without this, RenderMan will render them at their default width.
In Katana, they will have a geometry type of "pointcloud"
When RenderMan renders them, they will be rendered as disks or as spheres, depending on whether you have per-point normals.
See the RenderMan documentation for Particles to read more about how RenderMan renders particles.