How can I cross-list an article on the arXiv?
My account -> Change user information -> Group(s) you would like to submit to and you check the relevant categories. A hint might be (from the help): "Also note that you are unlikely to know that a cross-list is appropriate unless you are yourself a reader of the archive to which you are considering a cross-list". I had to write an e-mail to arXiv some time ago to learn how to cross-list.