Setting Custom Fields/Categories Programatically

We have a large list of computers (in the thousands). We would like to add some new dimensions (i.e categories / custom fields) to be able to search them based on different criteria (specifically when running task list or even to tune the roles and permissions). 

We know there's a manual method of going through each computer and setting the custom fields. That is more time confusing. Is there a more programmtic way to achieve this ? either via API or directly from the computer inventory itself somehow ? 

Thanks!