All attributes that are synced by Azure AD connect are passed from Active Directory to Office 365 including the the extension attributes.
A full list can be found here and we we are able to syncronise any other attributes you may required.
If you need any assistance please do not hesitate to contact our Technical Support Team:
IAM Cloud's Technical Support Team.
Support Portal: http://support.iamcloud.com
Email us: email@example.com
Phone (UK): +44 118 324 0000
Phone (US): +1 914 495 1298