In MS Word, there is special command in Home -> Paragraph -> Borders dropdown -> Horizontal Line, which inserts special drawing similar to 3D horizontal line in a paragraph. Such line should be exported to <hr> tag during HTML export, and respectively <hr> tag should be imported as Horizontal Line.

Note: the representation of the horizontal line in the docx document is as follows: 

        <w:pict>
          <v:rect id="_x0000_i1027" style="width:0;height:1.5pt" o:hralign="center" o:hrstd="t" o:hr="t" fillcolor="#a0a0a0" stroked="f"/>
        </w:pict>

Workaround: 
Currently, you can add a horizontal line using a table with a single border. Here is some code demonstrating how to do this:

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var document = new RadDocument();
var editor = new RadDocumentEditor(document);

Table table = new Table();
var topBorder = new Telerik.Windows.Documents.Model.Border(1, BorderStyle.Single, Colors.Red);
var borders = new TableBorders(table.Borders.Left, topBorder, table.Borders.Right, table.Borders.Bottom);
table.Borders = borders;

var row = new TableRow();
var dummyCell = new TableCell();
var dummyParagraph = new Paragraph();
dummyParagraph.FontSize = 0;
dummyParagraph.LineSpacing = 0;
dummyParagraph.SpacingAfter = 0;
dummyParagraph.SpacingBefore = 0;
dummyCell.Blocks.Add(dummyParagraph);
row.Cells.Add(dummyCell);
table.Rows.Add(row);
			
editor.InsertTable(table, shouldInsertParagraphBeforeTable: true);            
this.radRichTextBox.Document = document;

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