If I have a partition like /dev/hd1 that is unencrypted and want it to be encrypted, but want to keep everything currently in that partition, how can I do that?
There does not seem to be an solution to do that in place. Truecrypt offers the system encryption only for windows, dm-crypt overwrites partitions.
Your best bet would be to move everything from that partition into a backup with
Since this comes up near the top of google results, adding solution:
LUKS in place encryption via http://www.johannes-bauer.com/linux/luksipc/
Actually you can convert from a plain filesystem partition to dm-crypt.
But it's risky and cumbersome.
There is an out-dated tutorial here: https://help.ubuntu.com/community/EncryptedFilesystemHowto7
Dm-crypt maps one block to one block, so in theory it is doable. Luks is a user-friendly container that uses dm-crypt inside it. A luks partition contains a header and a dm-crypt partition inside it, where the encrypted filesystem really lives.