I can think of a very kludgey way to make this work by throwing some extra hardware into the picture, but the simple answer is no.
You can share a line appearance with an analog device connected to a VG200/2600/3600 POTS FXS port. But if you want it to ring an outside line, I think you would have to connect another router with an FXO port, with something like connection plar opx 95551212 or something in order to ring the outside number. But again that is not really a good way to do it.
What might be a better solution is to use a voicemail system that gives callers the option of trying a mobile number.
Or even better, something like Cisco Personal Assistant. That could be used to try a user on their work phone, then their mobile phone, other phone, etc.
