In most cases, occupation categories are considered novelty categories that do not provide much in the way of genealogical research. There are a few exceptions, such as following occupations through family lines where trades or skills are passed through the family; or where there is a one place study in a specific type of community, such as farming or mining communities.
At any rate, categories can be added by anyone. You just need to make sure that you follow the examples and conventions that are in use or make sure another category has not already been created for the same occupation, just worded differently.
If you are more interested in using these categories yourself, you may want to look into personal categories.
