You can try handling the TableControlCurrentCellStartEditing event, and if it is teh AddNewRow, then set the value at this point.
private void gridGroupingControl1_TableControlCurrentCellStartEditing(object sender, GridTableControlCancelEventArgs e)
{
Element el = this.gridGroupingControl1.TableControl.Table.CurrentElement;
if(el != null && el is GridAddNewRecord)
{
this.gridGroupingControl1.Table.AddNew();
Record r = el as Record;
int col = this.gridGroupingControl1.TableControl.CurrentCell.ColIndex;
int field = this.gridGroupingControl1.TableDescriptor.ColIndexToField(col);
int someParentID = 1;
r.SetValue("ParentID", someParentID);
}
}