There are several ways to go beyond a Grade 5 contrast, but they're all well beyond ordinary printing.
First, you can gain up to about a half grade, in some cases, by switching print developers -- unfortunately, the most common developers, like Dektol, are already the higher contrast sort.
Second, you could make a copy negative. Using either ortho lith film (can be handled under red safelight) or slow camera film (there are ortho films in this category, too), you can make a contact print positive, and from that a contact print negative, and control development in the process to get more contrast than the original. This method carries some cost, as you'll have to buy sheet film of the chosen type and probably expend several sheets on tests of exposure and development to get the correct final contrast.
Another option would be to intensify the original negative (I don't recommend altering the original in any way, but it's an option) -- this can be done either by bleaching and redveloping with a contrast-enhancing process, or using a process like mercury intensifier (warning! toxic chemistry!), or bleaching and toning the negative to produce increased density compared to the original silver image. There's also cost involved here, buying specialty chemicals and hazard mitigation (and possibly hazmat disposal of leftover chemicals), and you risk damage to your original negative.
A hybrid workflow option would be to scan the negative, enhance contrast in software, and then print the result (as a negative) to a transparency that you can then contact print. This last method is very commonly used to produce alt-process (cyanotype, Van Dyke Brown, kallitype, platinum/palladium, etc.) prints from small negatives originally processed at "normal" contrast for silver gelatin printing, but is also a valid method to obtain a desired contrast level for silver gelatin. Note that you should plan to contact print digital negatives, as otherwise pixels from the printing process will be visible in the enlarged print.
