Can my employer make me put them on my auto insurance?

Do you use your car for work--for example, to go to job or work sites, or to transport parts, materials, equipment, etc.? If you do, then your employer could face liability if you get into an accident on the way to/from a work location; an employer may be liable for accidents or injuries employees cause during the course of their employment. That would make their request to be put on your insurance, and to have the amount of coverage reasonable.
Furthermore, unless you have a written employment contract protecting or guarantying your job, you are an employee at will. That means that your employer has complete freedom to set the terms and conditions of work--and those terms and conditions can include putting your employer on your insurer. The law of employment at will presumes that if you do not like your job's terms and conditions, your recourse is to quit or change jobs.
Therefore, you employer can do this.
Note that independent of whether your employer requires this or not, if you do use your vehicle for work, make sure you tell your insurer that and buy the correct coverage. If you only buy coverage for personal use, but then get into an accident while using the vehicle for work, your insurer could deny coverage because you did not inform them of the commerical/business use (which changes the risk profile) and did not get the right insurance.