I think you need to simply use Add Data Row to the data table, then re-filter to your array of rows again and it will have that data row included. Now, saying that, I’m not sure if you are doing this inside a ForEach that references the rows, then you might need to restart the loop because the data table has changed at that point.
If you want to keep your loop, then you will need to duplicate your data table so you can change the data.
There are other ways too but you would need to calculate the row number in Excel (maybe dt1.Rows.Count+2, or whatever), and use Write Cell or Write Range with those coordinates. This solution would also require you to convert the column index to a char (like Char(65+index) )
So those are my thoughts.