Hello, i think i know what your trying to do, so basically if you have in let's say the discussion module 30 different categories, you then want to copy all those 30 categories to the post mod? If that's the case i don't think you can copy them over. I remember when building my site, i had to write all categories one by one to other modules. Perhaps Alexey or LeonidS can help you out further or Baloo 

Remember that before you make any core changes or modifications, back up your site.