How to find folders under site root using xml?

I wanted to add a multi dropdown button to a content item. In the dropwdown box I wanted to list the folders under site root.

For example, I have a site called //sites/HRSA with folders folder1, folder2 and folder3 on the root. The dropdown box should list all the folders folder1, folder2 and folder3. Instead of hard coding it, I wanted to create a xml app to pull the information from the folder tree. Is there a way to do it?