in order to Add Tree Node to RadTreeView it has to be of type RadTreeNode and yes it also accpets string as Text and also it has another constructor that accepts Text and value and below:
RadTreeNode result=new RadTreeNode(path);// that would be the Text of the Node
or//
RadTreeNode result=new RadTreeNode(path,"value");
// this constructor accepts first argument and the Node text and the second as the Node value
regards