Building new port, don't want to run as root

[Walter Venable]

Matthew Seaman wrote:
> I take it you're talking about a daemon process and you want to have the
> rc.subr scripts start it as another user than root?  That's fairly 
> simple.
>
> To make rc.subr start a process using a different UserID, all you need to
> do is define variables
>
>    name = foo                <-- standard rc script thing to
>                                            setup the namespace
>    foo_user = someone
>    foo_group = somegroup
Ok, silly question, but I looked around again and hit another brick wall 
-- how do I make the port add a user that doesn't already exist?