Can I encrypt an already existing, non-empty partition to use LUKS?
There is no converter, and it is not really needed. The way to do this is to make a backup of the device in question, securely wipe the device (as LUKS device initialization does not clear away old data), do a luksFormat, optionally overwrite the encrypted device, create a new filesystem and restore your backup on the now encrypted device. Also refer to sections “Security Aspects” and “Backup and Data Recovery”. For backup, plain GNU tar works well and backs up anything likely to be in a filesystem.
