So I have looked and looked, and it appears as if there is currently no way to do this.
I am on the CC subscription, meaning I have the latest version. At my school, they are not upgraded yet - meaning they are still on CS6. When I go to open or turn in my files for a project, the .indd files refuse to open - stating that the version they were created in (CC) is newer than what I am trying to open it in (CS6). I understand saving as an .idml file alleviates this issue, but what I really need is a convenient way to transfer all of the related links and fonts to the school computer as well. But as it turns out (at least as far as I can see) there is no way (currently) to package a file as an .idml file.
If I am missing something, please let me know. Otherwise, I suppose this is sort of a feature request. This is a tiny change that would help people in this situation quite a bit.
If you have a CC subscription you can also download and install CS6 (and run it in parallel) so you don't need to work in CC at all.
For situations like that I chronically save the INDD with the version in the filename, package, then export an IDML into the package. That should be easily done for a single file, and for many files it should be scriptable - but I don't bother, I just run Peter Kahrel's excellent Batch Export script twice, once for the package, once for the IDML.