Nested data in CSV files is not supported when importing data. CSV is meant to be a format describing flat data, and there aren't clear standard practices for embedding nested data.
The easiest way to index _geoloc attributes would be to write a script converting the CSV file into a JSON file and adding the _geoloc nested attributes to each JSON object. You can review the expected data formatting for enabling geo-search by adding geolocation data to records[1]
Links
[1]: Enabling geo-search by adding geolocation data to records