Simply put them in the grid, and then set the width of those columns to 0. It will add them to the export without making them visible to the user.
The only downside is that they're pretty hard to see in Studio Pro which columns you've already added without opening the details of the data grid.
The easiest way is to add them to the datagrid all the way on the right side and set their width to 0%. They wont show on the screen but they will be included in the export.
Alternatively you could make use of the excel exporter. To make more customized excel exports, this will require more work though.
Hope this helps!
Apart from the above two solutions, you could also try and hide the column using the following CSS.