Hi Alan,
DirSync only sync attributes from local AD to Office 365 (Online side), however the DirSync cannot sync attributes from online/Office 365 back to local AD.
As far as I know the proxyaddresses attribute is not stored in the "online AD"/Office 365 general "users and groups" tab. This attribute is saved in Exchange Online, so if you want to search this proxyaddresses attribute you need to write this application to access Exchange Online.
Since this is a third party application related question and we have little knowledge of this customized application, you can access this third party support and get some suggestions on the coding.
Thanks for your understanding.
Best Regards,
Bourne Zhang