In the help file for Centrex Transfer, it says:

Transfer from Voicemail/Auto-Attendant
This operation is only supported through Voicemail Pro using an Assisted Transfer action with the destination set to a Flash Hook short code.

But, I would try creating a short code for Flash, and in the Telephone Number field, put the outside number you want to transfer to. Create a phantom user, and unconditionally forward (including hunt group calls) to the short code.

Then have the auto attendant transfer to that phantom user