HELP: Howtwo create a passwd-suitable hash for usage with psswd -H 0?

Hello out there,

I run into a problem and digging for a solution didn't work out.

Problem: I need a string that reflects the hashed password for the usage with 

passwd -H 0

I think the procedure is using 

sha512 -s Password

and using this output for further processing, but how?

I looked for a possibility to emmit the hash created by passwd to be used, but
I couldn't find anything useful.

It is crucial that we use utilities available in the base system of FreeBSD, so
no fancy Python or Perl scripts.

I might have overlooked something in my desperate quest, so I'm sorry if the
task is trivial and already standard.

Kind regards and than you very much in advance,

oh