There isn't really a good method to create curved lines in SmartBuilder. I assume this is for visual header/footer backgrounds?
Probably your best bet is to use Illustrator or even PowerPoint to create images (probably a PNG, so you can make use of transparency), and bring those in to SmartBuilder using an image object. Depending on the nature of your design, you could even make a "negative" version, which has white sections surrounding a "knocked out" transparent section. And then you can layer a shape behind that, and change the color of the shape as needed.
Or you can get creative with the shapes SmartBuilder provides. If 2 shapes overlap, and they are the same color with no border, they look like a single shape. See the images attached for an example.