Azure AD Connect allows to extend the schema in Microsoft Azure AD with attributes from on-premise Active Directory. Not all Active Directory attributes are accessible in Directory Extensions wizard.
Provided scrips allow to manually add directory extensions to Azure AD and Azure AD Connect connector schema.
How to use:
Add-Extension-Property-To-Azure-AD.ps1 -ApplicationName "My application" -PropertyName property1
Add-Attribute-To-Azure-Connector-Schema.ps1 -AttributeName full_extension_name
Delete-Extension-Property-From-Azure-AD.ps1 and Delete-Attribute-From-Azure-Connector-Schema.ps1 scripts can be used to rollback changes.
Delete-Extension-Property-From-Azure-AD.ps1
Delete-Attribute-From-Azure-Connector-Schema.ps1
More information about directory extensions: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/connect/how-to-connect-sync-feature-directory-extensions.
Verified with Azure AD Connect V2 2.2.8.0 and PowerShell 5.1.14393.1944.