I'm afraid I'm not very familiar with the NEST client. It's a mapping configuration, so it needs to be set via the Put Mappings API. And it needs to be done before data is indexed to work properly (since ES needs to copy the data into that field during index-time)
You should be able to use NEST's mapping endpoints (https://www.elastic.co/guide/en/elasticsearch/client/net-api/current/auto-map.html#_manual_mapping) and copy the syntax from the prior link about the _all field.