If you do not have neither CIFS nor NFS licenses, you could create this directory using diag account.
Note: take care using this account.
First, enter in advanced mode:
filer> priv set advanced
Now, unlock and set a password to diag account:
filer*> useradmin diaguser unlock
filer*> useradmin diaguser password
Enter in the systemshell, create the directory you need and put the pubkey generated in the authorized_keys file:
filer*> systemshell
login: diag
Password: the same you set in the previous step
filer% mkdir -p /mroot/etc/sshd/root/.ssh
filer% vi /mroot/etc/sshd/root/.ssh/authorized_keys
filer% sudo chown -R root:wheel /mroot/etc/sshd/root
filer% sudo chmod -R 0600 /mroot/etc/sshd/root
Then, exit systemshell, lock diag account and exit advanced mode:
filer% exit
filer*> useradmin diaguser lock
filer*> priv set admin