CouchDB create a server admin via script
According to the documentation, you can create a new admin via the REST API by working with the _config endpoint.
By issuing a PUT request against /_config/admins/:username (where the request body is the password) a new admin will be created. (provided you have access to do so)
Futon is just using $.couch.config({...ajax options...}, "admins", username, password); to accomplish this.
As per http://guide.couchdb.org/draft/security.html, you can use something like:
$ curl -X PUT localhost:5984/_config/admins/myusername -d '"mypassword"'