I just leave a comment to the other users how NTextAtom can be used.
This issue was solved.

Thank you Nevron Support Team

Hi Junghyron,

Yes, that is also an alternative way to insert it (especially if you want to specify the tab width). Do you meet any problems with this approach?

Best Regards,
Nevron Support Team

For more information, I used NTextAtom as follows:
Where '\tab' means the tab in the text and it will be replaced to tab with width of 50.

// init find settings
NFindSettings settings = new NFindSettings();
settings.FindWhat = @"\tab";
settings.SearchDirection = ENSearchDirection.Forward;

// find all occurances
NRangeI textRange = NRangeI.Zero;
NSelection selection = view.EditingRoot.Selection;

while (view.EditingRoot.FindNext(settings, ref textRange))
{
  selection.SelectRange(textRange);
  NTabInline line = new NTabInline();
  line.TabWidth = 50;
  NTextAtom textAtom = new NInlineAtom(line);
  selection.InsertAtom(textAtom);
 }

 // move caret to beginning of document
 selection.MoveCaret(ENCaretMoveDirection.DocumentBegin, false);

Hi Junghyron,

You can use the following code insert a tab at the currently selected text selection:

richTextView.View.Selection.InsertText("\t");

If the selection object currently selects a range then the text will be replaced by a tab character. 

We hope this helps - let us know if you meet any problems.

Best Regards,
Nevron Support Team

I'm trying to replace the specific text (space or special text) to tab.
First, the text is selected by NSelection. 
But it does not have method for InsertTab.
How can I do this?