A common question among employees is whether your employer can and has a legal right to assign to you those job duties which are not part of your formal job description that you received upon hiring or at any time after you have been hired. The answer is usually (but not always) yes. If you are an at-will employee, and your employer can legally terminate you for any reason with or without notice, they surely can change your terms of employment, including your job duties at any time as with or without notice. And unless you have a contract with your employer that binds parties to specific terms of employment, including compensation and your job duties, the employer is not bound but the job duties they created for you. While the employer cannot literally force you to do those job tasks that you were not initially hired for, they can fire you for refusing to perform those additional job duties,and that kind of termination would be perfectly legal.
A related question is whether the employer has to pay more if they assigned to you additional or more challenging job duties. If you are an exempt employee, then they don’t have to pay more. It is up to you to decide whether you are willing to do those new and/or additional tasks for the same rate of pay. If you are an hourly, non-exempt employee, then the employer only has to pay more if you work more hours in order to complete those additional assigned tasks.