I am not sure if I can sign a message with these addresses now. but why do I need to do that? my account is not hacked. I can still login into my account with my 3years old password. I don't think I need to sign a message in this case.
You can login but can you access the account? It is not possible. The admin or moderators locked it and only them can unlock it. It is just normal for you to proof that you are the owner of the account. Those addresses are the addresses you have used before on this forum, if you can not sign any of it, that means you can not recliam back your account. Tomatocage stake address thread was meant for reasons like this.