How do I add an intro to a video without having to render the entire video again?
Using After Effects, I don't think there is.  But there are ways to make your life easier.
If you have a complex comp, if you've rendered the whole thing out to a format like ProRes (even though the files are large), it's relatively quick to drop in an extra intro and outro either side of your ProRes file, so you won't have to re-render the entire project again. You could also look at pre-renders to speed up your projects.