Well I know this can be done but do not know if it will be the same as before.
Assupmtions for the example:
Managers extension - 1000
1. Create a new partition called something like Boss_PT
2. Create a new Calling Search Space called something like ToAdmin_CSS and add the Boss_PT partition
3. On the Admin phone add extension 1000 (Mgr's ext) but put it in the new Boss_PT partition.
4. Forward this new extension to VM and use the Forward Calling Search Space ToAdmin_CSS
5. On the managers phone forward his extension to 1000 and use the Forward Calling Search Space ToAdmin_CSS
This should cause the managers phone to be forwarded to the admin after the defined number of rings.
Also you don't have to use the same extension as the Manager, you can use whatever you want.
You may also have to add the Voicemail partition (assuming you have a separate one) to the newly created Calling Search Space.
If the Calling Search Space for the device (as opposed to the line) is not set, you will need to set the Calling Search spaces for forward no answer, forward Busy, etc of the new extension on the admin phone.
I probably forgot something but give this a try.