If you have a smart phone you can download the 3CX mobile app (iOS / Android) and be on the go as your extension.
Otherwise, you can change your inbound routing to forward to external mobile. Depending on your current routing you could change the destination no answer to forward to your mobile.
You can either send calls destined for your extension to a mobile, or you can send all calls destined towards a DID to a mobile number.
If you are looking for a one-shot solution that you can switch on/off daily, consider using the Force IN/OUT of office function. You can dial a single code on any of the phones, and your whole system will divert calls to the destination of out of office hours. Beware though, you need to set it back to default hours afterwards otherwise it will remain in this mode.
On the other hand, if you temporarily need to divert all calls to a single mobile, you can do just like @kieferschild said and change your inbound routing to forward to external mobile.
Keep in mind that if the sets (extensions) are a member of a ring group, call forwarding will only work for calls direct to the extension, not to the ring group. In that case, using the Ring my Mobile at the same time option, on the extension, might be advised.