You can do it offline, but not on the site. If you only play in Greenfoot itself, you can change the user name and it will create a 'storage.csv' when you store something. From this, you can get the names/scores/ints which you need to create your highscores. The 'storage.csv' is like the offline version of the file which is saved online and can be viewed through the 'get data' button in your own scenarios on the site.
They are not connected, the 'storage.csv' is only for local use.
and how would i make highscores with this storge.csv
The offline storage spreadsheet works with the same code as the online storage database. It was created that way to simulate the online storage of data, so you could test it for online storage without have to upload it a bunch of times before getting it right. You can change the apparent user, offline, through 'Edit>Preferences -- Miscellaneous' via the menubar (or 'Ctrl+Shft+P').
BTW, offline, greenfoot will create the storage file in your scenario folder the moment any data is saved, if it does not yet exist.