Create secondary alias in à create mailbox policy


I want to be able to add a secondary email adresse to all my new user account so that I can migrate them to my Office365 during the night.

I have a policy that creates new exchange account.  I tried to add a new secondary email address to the account with the policy option "E-mail Alias Generation" but it doesnt work.  I keed getting only the default Alias.

I tried with the policy option "Property Generation and Validation" and when I whant to change the value for the attribute proxyadress I always get some errors.

Do you have an easy way to add a secondary email?