Yes. One easy way to do this is to create an encrypted disk image using Disk Utility on OS X or using Windows Explorer’s file attribute option on Windows (not available on Windows XP Home Edition). A variety of third party software for creating encrypted files and folders will also work. A great free, cross-platform (Windows XP & Vista, OS X, Linux) option is TrueCrypt.
Encryption tools that try to encrypt every block on Drobo will not work unless you create a dedicated partition for storing encrypted data. If you do not create a dedicated partition these type of tools will try to encrypt every block on Drobo, but never finish since Drobo creates a virtual disk partition large than available disk space. Creating a dedicated partition that is the same or smaller than the available capacity actually installed in Drobo avoids this issue.
Drobo is currently not compatible with hardware-based Full Disk Encryption (FDE).