Adding directly attributes to entities from runtime (client side) isn't possible. This can only be done in the model.
But you can make it dynamic by adding a new entity with association 1-* to the account entity:
Add a listview to the required page;
And the result will be something like this;
Ofcourse you need to take care of the creation of the referenced objects. Either manually, or generated automatically based on a template or such.
In addition to Rene's answer, you can also create another object called 'Values', which has an association to ExtraField and Account. This enables the admin to add ExtraFields that can be used with every instance of the Account object.