I din undrstnd the actual requirement of urs but still juz check this if u wanted something like this::
Private Sub btnAddAtFirst_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles btnFind.Click
Dim objtextRange As TextRange
Dim strVersion As String
Dim objMessageParagraph As Paragraph
objtextRange = New TextRange(cTextBox.Document.ContentStart, cTextBox.Document.ContentEnd)
strVersion = objtextRange.Text
strVersion = "Hello" & " " & strVersion
objMessageParagraph = New Paragraph
objMessageParagraph.Inlines.Add(strVersion)
cTextBox.Document.Blocks.Clear()
cTextBox.Document.Blocks.Add(objMessageParagraph)
End Sub