Yes, but it's a little messy if you want to do this for everyone. For example, rather than forwarding calls directly to voicemail (on no answer, busy, or not registered) you could forward calls to a Digital Receptionist (DR). The DR could say something like "I'm away from my desk.... to contact me on my mobile press 1, or to leave a voicemail message press 2". Naturally, when they press one, the DR would forward to the cell number. When the caller presses 2, the DR would transfer directly to the correct voicemail box. It's pretty straightforward, provided, that you don't need this capability for all your users. If you have a large number of extensions and want to implement this feature for everyone, you would have to create a DR for each and every person.
As an alternative, If you want to implement this for a large number of users, you could forward calls to a CFD application. You can capture in the CFD the transferring extension, so you know where the call is coming from, then you could look up the extensions mobile number using the Call Control API, or transfer directly to the referring extension's voicemail box. A little work, but far less messy if you have a large number of extensions that need this capability.