I see what you mean; we’ll make a change to allow new line characters in the String Node. It should go out with our next release.
In the meantime, you can use a Function Node to accomplish what you’re looking to do. Assuming your current string is stored at the payload path of data.myString, you can concatenate a new line with this code in the Function Node:
payload.data.myString += '\nThe String to Concatenate';