I don’t think we want the President of the United States to have a say in who gets hired on a local police force.
But just for the sake of argument let's say the FBI decides they want to be able to move into large cities in emergencies - and ‘hit the ground running’... meaning they need people in local police forces ‘FBI trained’... something like a ‘top cop’ concept...
Then let's assume that someone like Peter Strzok is running the FBI - and choosing people of 'like mind' to work with him... and those people make similar choices with their staffs. A liberal/progressive/commie mentality becomes 'the culture'... and all that goes with with that culture. In short - over time it filters down to local police departments.
When Trump chooses conservatives for the FBI and encourages them to choose conservatives it will eventually effect the ranks. Same with conservative judges and the court system... Trump needs to choose carefully because five years down the road a person promoted to chief could likely be a conservative.