To hijack or steal a Mobile Suit in the story mode, you must end a mission with an enemy Mobile Suit still alive but with some damage. Chances are that if it is the only Suit besides you and your ally, you will see a message similar to "You have received a stolen GM". You can use it for ground combat." To get the hijacked Suits for arcade and the versus mode, kill one in arcade mode and survive the battle in which you killed it. When you open the versus mode or arcade mode, it will be there, ready for duty.