The Marketing permission in Leads do not allow to add any extension fields.
Reproducing the Issue
Go to Sales work center.
Go to the Leads view.
Open Lead XYZ (XYZ represents Lead ID).
Go to the Marketing facet.
Go to the adaptation mode.
Edit Master Layout.
Add field selection.
Here the option to add New Field is not there.
This is beacuse we have not enabled the extension scenario for Mareketing Permissions in Leads yet.
This is planned in 1805 release and currently we have the Marketing Permission extension scenarios enabled only for Contacts and Individual Customers.
Marketing Permissions, Leads. , KBA , marketing permission , extension fields in leads , LOD-CRM-LM , Lead Management , Problem