Hi,
regarding BoxCryptor for Windows, you can simply copy the whole encrypted folder to your Insync folder and change the BoxCryptor preferences to use the new location.
But indeed, you would loose support for mobile devices. Our mobile apps only work with Dropbox right now, but we're planning to integrate other services also. You can add such a feature request to our feedback forums, so that other users can vote on it:
http://boxcryptor.uservoice.com/
--Robert
