<Window x:Class="WpfApp7.MainWindow"
        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"
        xmlns:local="clr-namespace:WpfApp7"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ContentControl x:Key="MyContent">
            <StackPanel>
                <CheckBox Content="Item1"/>
                <CheckBox Content="Item2"/>
                <CheckBox Content="Item3"/>
                <CheckBox Content="Item4"/>
                <CheckBox Content="Item5"/>
                <CheckBox Content="Item6"/>
            </StackPanel>
        </ContentControl>
    </Window.Resources>
    <Grid>
        <Button Padding="10,5" HorizontalAlignment="Center" VerticalAlignment="Center"
                Content="Open Dialog Window" Click="Button_Click"/>
    </Grid>
</Window>

using System.Windows;

namespace WpfApp7
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var dialogWindow = new Window { Content = FindResource("MyContent") };
            dialogWindow.ShowDialog();
        }
    }
}