Synchronized objects in Active Directory, we can not make any changes in Exchange Online.
To add a user for secondary SMTP example:
charles.santana@supportbrazil.com as primary SMTP and
csantana@supportbrazil.com as secondary SMTP
Access:
Then select the object:
Right / Properties / Attributes
Locate ProxyAddress
add secondary SMTP:
SMTP: csantana@supportbrazil.com
Ok in all screens can force synchronization or wait for the normal period synchronization of Exchange Online, it also serves to distribution groups.
Charles Aureliano Santana 