Yes, in fact we require it in the following circumstances:

• To accommodate a customer of size who encroaches on an adjacent seat area and/or is unable to sit in a single seat with the armrests lowered;
• To transport a customer who, because of his or her particular disability, would be unable to travel without the purchase of additional space on the aircraft;
• To transport large musical instruments or electronic audio/video, medical, or other sensitive equipment unsuitable for carriage as checked baggage, as specified in the Contract of Carriage

You can purchase an extra seat by using your name for both tickets and selecting the desired seat assignments.