Plan9's ssh client supports only the SSH v1 protocol, If you simply must have the V2 protocol, a port of OpenSSH exists (see Contrib index), or you can volunteer to finish the native SSH v2 client - work on which has stalled.
If SSH v1 is ok you can use auth/raskeygen to create a key for yourself.
Assuming the remote system is called unix:
auth/rsagen -t 'service=sshserve owner=*' > key auth/rsa2ssh key | ssh unix 'cat >>.ssh/authorized_keys' cat key >/mnt/factotum/ctl
You may now want to use ipso(1) to install the key in your secstore.
For more information see rsa(8)