Sure you can, although you'll have to disable the timer, after which they are set to self-destruct. You won't be able to save them to "Saved Messages," though, because the "Forward" option is disabled for these types of texts. You can, however, archive Secrets Chats and add them to folders you've created manually. In fact, once you select a particular chat thread to be added to the new folder, Telegram will automatically add any Secret Chats you might have initiated with the person in question (on mobile, where the option is available).