You created an extension field ABC (ABC is the label of your field) under Appointments and would like to add it to Outlook Add-In, but there is no option to add this field to the add-in.
SAP Cloud for Customer.
Currently this is the system expected behavior, there is no option to add fields to Outlook Add-In Client Side.
You can also submit your feature request on our Ideas Place, that is the official tool to propose new functions and share scenarios like this.
Outlook Add-In, Add fields to add-in, Add-In. , KBA , LOD-CRM-GW-OUT , Outlook , Product Enhancement