Are you using the eBookwise Librarian software to send files to the reader? If so, you can assign any category to an .imp file by clicking "Details" on the librarian screen. But you must do this before you upload the file to the reader. Once it's sent to the reader you can't change or assign categories.
BTW, the Librarian software is still usable for uploading files even after the trial period expires. It just limits the number of files in the listing sent to the reader to three.