Every admissions officer I have dealt with says they got involved in the field as an undergraduate working in the admissions office or giving tours, and then staying on after graduation as a paid employee. These are the traveling adcoms that visit schools or college fairs recruiting kids to apply, and the various adcoms you meet while touring campuses. They are also the level of adcom that does the first reads and first weeds of applicants.
If you can provide some evidence that the primary method that admissions officers enter the field is not as I have described, please do so.