You cannot have "free lines" in yEd. However, you can fake a "free line" by connecting two invisible nodes. The easiest way to create an invisible node is to create a simple rectangularnode and set its "Fill Color" and "Line Color" to "No Color".
How easy it is to customize a round rectangle with additional lines depends on what lines you need. Can you upload a mock-up that shows what you want to achieve? (To create a mock-up, create a round rectangle in yEd, export to an image format such as JPG or PNG, open the image in an image editing application such as the GIMP, Microsoft Paint, or Adobe Photoshop, and add the desired lines.)