Hello,
I am not quite sure about your requirement. As far as I know we cannot set the level of the RadTreeNode when adding the node.
If you want to add a node to a particular level then you can loop through all the nodes and check for the level using
get_level() method and then add the node. Checkout the following documentation link which describes the most important functions of client-side RadTreeNode object.
RadTreeNode
RadTreeNodeCollection
Thanks,
Princy.