export the table "users" as CSV-File from your database.
Have a look at the BwPostman manual - section "Import and Export" and at the file import_demo.csv (since version 1.3.0 you can also use the fields "gender" "and "special").
You can find "import_demo.csv" in the directory your-site-root/administrator/components/com_bwpostman/ or
extract the downloadpackage of the component and navigate to the folder admin/.
Edit your CSV-File and go to joomla backend "Components->BwPostman->Subscribers->Import".
Import the users as subscribers.