You can specify the Microstation seed file that will be used for exporting.
On the Seed File tab of the Modify DGN Export Setup dialog (see Export to DGN), you can view the Microstation seed file that will be used for exporting. To specify a different file, click 
(Modify Export Setup), navigate to the desired file, select it, and click OK.
You can also specify whether to use Master units or Sub units as the working units.