Does anyone know whether the Kindle can be connected natively to any other cloud based storage solutions than Amazon Cloud?

I would like to have the CLOUD icon in the "bookshelf" screen point to Dropbox (as that's where I have all my PDF files). Also would like to have all the nice book \pdf icons preserved.

Is this possible? I have read that you there are alternative ways of conencting to cloud storage such as installing File Manager \ Drop Box apps but these do not "natively" connect to the CLOUD icon in the bookshelf screen.

Any ideas?