If the update is an MSI you cannot apply the update to the running virtual environment. Windows Installer service runs outside the virtual bubble and therefore updating is not possible. If the update is not using Windows installer you should be able to capture the update into the sandbox and then run sbmerge.
One method sometimes possible is to run Setup Capture and capture only the update (in this case IE8 locally installed). Investigate the project folder and manually merge the changed files into existing project. But this is not really possible if the update changes much in the registry.