I believe you will need to use Allegro Package Designer (APD) to export GDSII format. In APD, you can import a .brd file (Allegro PCB file format). Once inside APD, one of the File-> Export options will be GDSII.
You want want to make any changes while in APD because if you add database objects not supported by Allegro PCB, you will not be able to export the file back to a .brd file.
I'd like to provide you a suggestions.
ACCPCB is a specialized manufacturer for professional PCB in China.
It can produce single-sided,double-sided,multilayer PCB.
The point I'd like to let you know is that ACCPCB also provide PCB technical support. If you have any difficulties in getting, please be free to contact in the following: