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)