Sadly, no, you can't....
Tested with TrueCrypt 7.1a: If you go to
'Create Volume' > 'Encrypt a non-system partition' > 'Hidden volume' you have two options:
Normal mode: You have a nice:
WARNING: Note that the partition/device will be formatted and all data currently stored on it will be lost.
If you select this option, you will create a hidden volume within an existing TrueCrypt volume.
The latter case is not a choice, since the partition is not yet encrypted.
So, you can't! At best you could encrypt the partition in normal mode, then create a hidden volume within it, and then transfer the data from the normal volume to the hidden volume... but since you can't resize a hidden volume, you will loose the size of your data in drive space. Certainly a bad choice. Moreover the normal volume would have to be wiped after this operation to preserve plausible deniability.
So the best choice is sadly to copy everything to a temporary location (provided that you have the space to do so), create the hidden volume, and finally to copy the data back to the partition. Don't forget to wipe to temporary location or to use a temporary truecrypt volume in this temporary location.