It is actually possible to get ITAR exceptions for candidates who aren't US Persons. I know there are engineers at SpaceX from the UK, Canada, and I'm pretty sure Australia as well. It is a pretty long and expensive process from what I understand so it is used only for those with very specific skills or knowledge. I'm sure it would also be quite a bit harder or impossible for candidates from China/Russia or less "friendly" nations.

This of course isn't to imply that SpaceX has a monopoly on talented engineers, I'm sure they exist all over the world.