Hi,
It is not possible to do it silently. It is not secure, because any app from market can include some another app with different permissions that easily can be a virus-app.
But it is possible to initiate installing of some apk that is located in external storage (sd card), where you can copy it beforehand from the root app. User will be asked if he want to install the apk or not. He can always decline.
If this is a way for you, I can make it.
Thanks,
Alexey.