Introduce Split Table Command. 

In MS Word, the command is in Table Tools -> Layout -> Merge -> Split table.

Workaround: Use the RadDocumentEditor API to split the table:
 
var tableRowBox = this.radRichTextBox.Document.CaretPosition.GetCurrentTableRowBox();
if (tableRowBox != null)
{
    this.radRichTextBox.BeginUndoGroup();

    this.radRichTextBox.Document.Selection.Clear();

    var currentRow = tableRowBox.AssociatedTableRow;

    DocumentPosition start = new DocumentPosition(currentRow.GetRootDocument());
    start.MoveToStartOfDocumentElement(currentRow);

    DocumentPosition end = new DocumentPosition(start);
    end.MoveToEndOfDocumentElement(currentRow.Table.Rows.Last);

    this.radRichTextBox.Document.Selection.SetSelectionStart(start);
    this.radRichTextBox.Document.Selection.AddSelectionEnd(end);

    this.radRichTextBox.Cut();

    this.radRichTextBox.InsertParagraph();
    this.radRichTextBox.Paste();

    this.radRichTextBox.EndUndoGroup("Split table");
}