Paragraph par = new Paragraph();
...
rtb.Selection.Insert(par);

        public static readonly DependencyProperty IsBubbleSourceProperty = DependencyProperty.RegisterAttached(
          "IsBubbleSource",
          typeof(String),
          typeof(Paragraph),
          new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.AffectsRender)
        );
        private void InsertNewTable(uint Rows, uint Colls)
        {
            Paragraph par = new Paragraph();
            par.SetValue(IsBubbleSourceProperty, "значение");
            InlineUIContainer container = new InlineUIContainer();
            par.Inlines.Add(container);
            oTable oT = new oTable();
            container.Child = getDataGrid();
            rtb.Selection.Insert(par);
            ReturnFocus();
            MessageBox.Show("!!!!");
        }

            DependencyProperty MarginProperty = DependencyProperty.RegisterAttached("Margin",
                typeof(String), typeof(Paragraph), null);
            Paragraph par = new Paragraph();
            par.SetValue(MarginProperty, "_______________________________________________________________________________");
            rtb.Selection.Insert(par);
            ReturnFocus();

<UserControl x:Class="SlTextW.InputPar"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">


    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.RowDefinitions>
            <RowDefinition Height="43*"/>
            <RowDefinition Height="257*"/>
        </Grid.RowDefinitions>
        <Button Content="Button" Visibility="Visible" HorizontalAlignment="Left" Margin="149,0,0,10" Width="75" Height="22"
                VerticalAlignment="Bottom" Click="Button_Click"/>
        <RichTextBox x:Name="rtb" AllowDrop="True" BorderBrush="{x:Null}" Margin="10,10,10,7"
                TextWrapping="Wrap" VerticalScrollBarVisibility="Auto" FontSize="20" Grid.Row="1" />

    </Grid>
</UserControl>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace SlTextW
{
    public partial class InputPar : UserControl
    {
        public InputPar()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Paragraph par = new Paragraph();
            DependencyProperty MarginProperty = DependencyProperty.RegisterAttached("MyValue",
            typeof(String), typeof(Paragraph),null);
            par.SetValue(MarginProperty, "++++++++++++++++++++++++++++");
            rtb.Selection.Insert(par);
        }
    }
}

        <RichTextBox x:Name="rtb1" Margin="10">
            <RichTextBox.Resources>
                <Style TargetType="Paragraph">
                    <Setter x:Key="KeyMargin" Property="Margin" Value="0" />
                </Style>
            </RichTextBox.Resources>
        </RichTextBox>