Thank you for writing.
I am not exactly sure what you want to achieve. I would give you a few solutions for different cases. If none of them covers your case I would kindly ask you to provide a detailed explanation of your scenario so I can help you find the best approach in the situation.
1. If what you want to do is to add new entries to the data table, you should directly add them in the existing data table and all changes will be reflected in the RadGridView automatically.
2. Taking into account that DateTable.Clone() only copies the schema of the table, if you want to clear the records and add new ones you should use the Clear() method on the original data table and then fill it with the new data. Again all changes will be reflected in the RadGridView.
3. If you want to keep the data in your original table and only show the new one in the grid you should set the new table as DataSource for the RadGridView, but since you set the new table to the original one I guess this is less probable.
4. If you want to make all the addition away from the RadGridView so it would not update itself on every row added you can surround the addition of rows with a RadGridView.BeginUpdate() and RadGridView.EndUpdate() calls.
I hope this will be useful for you. Should you have further questions, I would be glad to help.
All the best,
the Telerik team