You can try handling CurrentRecordContextChange and set the defaults there if you are starting to edit a record.
private void gridGroupingControl1_CurrentRecordContextChange(object sender, Syncfusion.Grouping.CurrentRecordContextChangeEventArgs e)
{
if(e.Action == CurrentRecordAction.BeginEditComplete && e.Record is GridAddNewRecord)
{
GridAddNewRecord rec = e.Record as GridAddNewRecord;
rec.SetValue("Col0", "Col0Default");
rec.SetValue("Col1", "Col1Default");
rec.SetValue("Col2", "Col2Default");
rec.SetValue("Col3", "Col3Default");
}
}