Synchronized objects in Active Directory, we can not make any changes in Exchange Online.
To add a user for secondary SMTP example:

[email protected] as primary SMTP and
[email protected] as secondary SMTP

Access:

Then select the object:

Right / Properties / Attributes

Locate ProxyAddress

add secondary SMTP:
SMTP: [email protected]

Ok in all screens can force synchronization or wait for the normal period synchronization of Exchange Online, it also serves to distribution groups.