Just about everything on their menu is customizable. It may not give you options for what you can do with what they offer but I've been able to create my own juice/smoothies mixing fresh fruit items off the menu or even one of the traditional milk tea flavors with some fresh fruit and tapioca. There usually isn't a surcharge for using more than one fruit in a slush etc and they're cool about making it less sweet (recommended, because they add a ton of simple syrup)
*The regular milk tea flavors on the first menu are powders and are very sweet and artificial so just keep in mind it's not particularly healthy and it's packed full of sugar
I prefer lighter flavors like honeydew with watermelon fresh slush/smoothie.