AlternateEmailAddresses and ExtensionAttribute

Looking to get the latest on this, lots on the web but my search showing dated info.

I do not have exchange enabled on my tenant at this time.

In my internal AD, ExtensionAttribute3 has my users alternate email address.

Am I still correct in saying because ExchangeOL is not configured I cant get to/utilise my msolusers extensionattribute's (via powershell or any other method)?

i.e. I can't take extensionattribute3 and write it to AlternateEmailAddresses within AzureAD, my only alternative is to get the value from my AD and write it to Azure AD (isn't that ADConnects job?)

Thanks,

Aengus