Hello,

Unfortunately  the area you want to re-template is not part of GroupByRecordPresenter and in order to add the margin you mentioned and achieve similar behavior as from the attached image, I suggest to handle RecordExpanded and RecordCollapsed events of XamDataGrid.

You can get the current expanded(collapsed) record from EventArguments and set the Margin you wish for record above and below it. It’s easy to find those records  if using GridViewPanelFlat. ChildElements collection: 

private void xdg_RecordExpanded(object sender, Infragistics.Windows.DataPresenter.Events.RecordExpandedEventArgs e)
{
    GroupByRecord groupRecord = e.Record as GroupByRecord;
    if (null != groupRecord && groupRecord.IsExpanded)
    {
        GroupByRecordPresenter groupRecordPresenter = (GroupByRecordPresenter)GroupByRecordPresenter.FromRecord(groupRecord);
        foreach (var item in (groupRecordPresenter.Parent as GridViewPanelFlat).ChildElements)
        {
            if ((item.GetType() == typeof(GroupByRecordPresenter) && (item as GroupByRecordPresenter).Record.Index == groupRecordPresenter.Record.Index - 1)) //presenter above
                (item as GroupByRecordPresenter).Margin = new Thickness(0, 0, 0, 15);
            else if ((item.GetType() == typeof(GroupByRecordPresenter) && (item as GroupByRecordPresenter).Record.Index == groupRecordPresenter.Record.Index + 1)) //presenter below
                (item as GroupByRecordPresenter).Margin = new Thickness(0, 15, 0, 0);
        }
    }
}

private void xdg_RecordCollapsed(object sender, Infragistics.Windows.DataPresenter.Events.RecordCollapsedEventArgs e)
{
    GroupByRecord groupRecord = e.Record as GroupByRecord;
    GroupByRecordPresenter groupRecordPresenter = (GroupByRecordPresenter)GroupByRecordPresenter.FromRecord(groupRecord);
    foreach (var item in (groupRecordPresenter.Parent as GridViewPanelFlat).ChildElements)
    {
        if (item.GetType() == typeof(GroupByRecordPresenter)) //presenter above
            (item as GroupByRecordPresenter).Margin = new Thickness(0, 0, 0, 2);
    }
}

I have attached simple sample application, where you can test the suggested approach.

Please let me know if you have any questions regarding this matter.

Sincerely,
Zhivko
Entry Level Software Developer