Getting power straight form the power truck is the smartest way as some lamps electronically controlled and cannot supply the extra current.
As for extra components, I presume the Christmas lights are DC powered, therefore you would need an adequately sized bridge rectifier and possibly appropriately sized current limiting resistor. The sizing of these aforementioned components are contingent upon the electrical specifics of the Christmas string. Unfortunately, the website listing does not share much so you'll have to take some experienced based guesses or contact them for their recommendations.
Your solution, with their guidance, may actually turn out that you use the rectifier from Evan Designs and the 3V DC version of their Christmas Lights