Yes, you can upgrade an existing pool's on-disk-format to support encryption, by running:
zpool set feature@encryption=enabled testpool
Or to enable all new on-disk features, simply do:
zpool upgrade -a
However, you cannot turn on encryption for a pre-created filesystem: you have to use
zfs create -o encryption=on <filesystem> at the time of creation in order to encrypt your data. If you want to start using ZFS now, you'd have to create a new filesystem after upgrading and copy existing data from your unencrypted (old) dataset into the (new) encrypted one.
This and many more ZFS encryption administration topics are covered very nicely in this article.