Greetings from Syncfusion Support.
Query 1: Please let me know how we can add new sub node as the add button only adds a node in the first level of nodes.
In treeGrid, we can able to add the new record at child node by setting the new row position as “child”. Kindly refer the below help documentation link,
Query 2: Also is there any other control that may have this feature.
We can able to achieve the same in Gantt control also, by setting the new row position as “child”. Refer the below API link,