Exporting can be done from a single PDF file to an FDF file. You can then use an Action (in Acrobat Pro) to import this FDF file to other files, which would create the comments you exported from the original.
To do the second part you can use this script:
(replace the file path with the actual location of the file you saved, by using the same syntax!)
Then add a Save command to the Action and run it on your files, and you're done!