There might be a way. I ran across this screen in my Kindle account. It will allow you to add media items you didn't purchase at amazon if you can provide an ASIN, ISBN or UPC code.
I don't know if you can items to the Kindle library specifically but you can add them to your overall media library. I've noticed that the overall library does include books I haven't purchased at Amazon, they were books I indicated I already owned in the recommendations list.
Maybe someone with some mobi books who owns a Kindle can experiment.
ETA: On second thought, if you want to upload a book from another source to be able to store it at Amazon for future download, I don't think this will do it.