We can do the category import for you.
But that requires a category export which is not the same as the export of your user data.
You could ask the owner of the original forum if they would be willing and able to provide you with a category export.
IIRC that would also contain the user profiles of other users who participated there (and that would end up in your forum as well). I am not sure if that is something you (or those users, or the owner of the other forum) want.
My estimate is that in this specific case it is easier / less work to manually copy/paste the contents.