Shouldn't be a big problem.
Lets say extensions at site A are 2XX, and site B are 3XX. We'll call the guest mailbox at site B 333.
Build a target line on site B with received digits of 333. Set the prime set of that target line to the voicemail DN. In mailbox 333, set the target line you just built as an express messaging line.
Build an external autodial with digits 333 on the operator phone at site A. Call comes in, she can transfer to 333. The call hits site B and goes directly to mailbox 333. You may need to enable 'allow redirect' on the operator phone in order for that to happen. You can test it first and see if that's needed or not.