NPOs often hire outside of their organization for grant writing work. Writers who are contracted to do grant work (or any other work to generate publishable materials) are generally paid in a lump sum with no explicit right to monies collected as a result of their work.
Grant writers are available for hire in many fields. Students or recent graduates are often willing to take on a stipend wage for the experience and resume reference. Freelance writers are another labor pool that is available to do grant writing. Finding workers to write grants can be a relatively easy task because they can be found and hired online. They don’t have to live in the same state as the NPO or be a member of its constituency to take on a job.