There's currently no possibility for expanding an existing model besides stretching existing elements and then refining them, which is only a workaround for small extensions. If you know that an extension will be necessary later, you could think about creating a larger mesh from the beginning, adding one or more additional superelements. Before doing the first simulations, you can then delete the additonal part. Such a procedure would make sure that for the second stage of the project (with extension) the elements are exactly the same as for the first stage.