Ypu can do that. Or try to add the one with the Ip first, if it does not work, change it to the hostname. Dont forget to reload mysql or flush priveliges after you changed a user.
Since this user need have full access on db.server.com, i would set to use the ispconfig login instead make a new one?
That depends on your setup. If the db server is not the amster server, then you can reuse it. If the master server is not the db server, then create a new user as the one created for the ispconfig setup allows only access from a slave to the master and not from one slave to another.