The functionality that you are trying to achieve requires a modification in the database. TreeList uses a so-called self-referencing hierarchy technique, this is a structure where you are using single table of flat data but because of the rule which is implied in the data you could display it in hierarchical structure. The rule is simple you have two fields with relations, ID and ParentID for example, one item is set as a child to another parent item based on the values of these two fields. For example if you have four items with ParentID equal to five then these four items will be displayed as child items to another item that has its ID property equal to five.
Keeping this in mind when you want to change the structure of your RadTreeList instance you need to modify the values of these fields in the datasource. So, the approach will not be easy and if your solution do the job you may want to stick to it.
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