You can create additional private datasets for an existing boot environment by using the zfs command to create a dataset located hierarchically under the root dataset of the boot environment.
This new dataset will not be automatically cloned to existing, associated zone boot environments. Also, because the new dataset is under the root dataset of a boot environment, the new dataset will be private to that boot environment. The new dataset will not be a shared dataset.
Once this new dataset has been created under the root dataset, if you then clone that whole boot environment, the new dataset would be included in the new, cloned boot environment.
For example, to create a new, non-shared dataset mounted at /myfs for the boot environment, BE1:
# zfs create -o canmount=noauto rpool/ROOT/BE1/myfs
For further information, see the zfs (1M) man page.