It is possible to add additional columns to the excel export if they are currently not defined as columns in the datagrid.  To do this, you need to override the method am_OnDataPresenterCreated in the content window code behind:

protected override void am_OnDataPresenterCreated()
{
     base.am_OnDataPresenterCreated();

     var typedDataPresenter = ap_DataPresenter as AB_DataPresenterBase;
     typedDataPresenter.ap_ExcelColumnSelector =
     (datagrid) =>
         {
            var columns = typedDataPresenter.am_BuildColumnDictionary(datagrid);
            columns.Add(CustomerEntity.EmailProperty.ap_PropertyName, CustomerEntity.EmailProperty.ap_Description);
            return columns;
         };
}

Use the columns.Add method to add each column. The key should be the entity property name and the value should be the entity property description.