You can use template columns in the datagrid. Perhaps a template column with a larger textbox might work.
Otherwise, I'm not sure if you can do what you are asking. The datagrid is a pretty rigid control regarding how it renders itself from what I have found. You can make all kinds of changes to the columns of one row, but adding a row is another story.
Perhaps you might be better off to try this:
Manipulate your data source before you bind to the grid. Add rows to the source data (which is MUCH easier). You can flag the row somehow to indicate that it's the "comments" row. Then you can deal with the row in the grid's ItemDataBound event handler. Make whatever column changes you need in that row. The grid will actually help you out with flagging the row because all the rows you add on (one comments row for each data row) will actually end up being the grid's 'Alternate Item' (versus 'Item'). So then you just need to manipulate the comment row's columns to fit your needs.