WPFに外部アプリ（Window）を埋め込む

MainWindow.xaml.cs

var psi = new ProcessStartInfo(@"program_path");

var process = Process.Start(psi);
process.WaitForInputIdle();

System.Threading.Thread.Sleep(3000);
SetParent(process.MainWindowHandle, _panel.Handle);

MainWindow.xaml

<WindowsFormsHost>
    <forms:Panel x:Name="_panel" />
</WindowsFormsHost>