well, I guess it is possible to manually create a user that has access to everything under var/www ?
No, at least not one that will work. You can create a user with root priveliges that has access to all files or use the root user. But as soon as you use the user to upload files, you will have to chown every file and folder to the owner of the web afterwards. If you dont do this, then suexec and suphp wil deny access to these files.