Such a technique is called steganography. It's possible but would require lots of bandwidth depending on how secure you need it to be. For example you can hide data in a photo by slightly changing the shades of red in it without changing the appearance of the photo noticeably.
I'm aware of steganography, but this would be slightly different than any published steg technique that I am aware of, as it would not be hiding in a preexisting carrier signal, it would be creating its own.
There are a lot of government supported efforts to research (especially finding and cracking) steganography. You can even look up articles in Chinese on the topic
