Go to the Character Customizer and create the proper costume. Use a human head, the appropriate red clothing...and don't forget the red cap! Santa can wield the weapon of your choice -- I recommend the green or red lightsaber to seasonally complement his shiny red duds. (Remember, the color of his lightsaber decides whether his Force powers are naughty or nice.)
In the Extras menu, activate "Disguise 3".

Password Effect
CL4U5H for Santa hat and red clothes
TYH319 for white beard (In Extras)