This is difficult to implement. Since RoundCube is not a webmail administration script, I doubt it will be a default feature; however, there are already some users who have created password change scripts for certain servers (cPanel and/or Plesk I believe). You should search the forum to see if perhaps there is already one out for your server. But since there are a bunch of email servers, and only one RoundCube, I don't think RoundCube will actually put this feature in as a default (although it is possible).