use Symfony\Component\Serializer\Context\Encoder\CsvEncoderContextBuilder;
use Symfony\Component\Serializer\Context\Normalizer\DateTimeNormalizerContextBuilder;

$initialContext = ['custom_key' => 'custom_value']);

$contextBuilder = (new DateTimeNormalizerContextBuilder()
  ->withContext($initialContext)
  ->withFormat('Y_m_d')
  ->withTimezone('GMT');

$contextBuilder = (new CsvEncoderContextBuilder())
  ->withContext($contextBuilder->toArray())
  ->withDelimiter('-')
  ->withHeaders(['foo', 'bar']);

$this->serializer->serialize($data, 'csv', $contextBuilder->toArray());
// Serialization context will be:
// [
//   'custom_key' => 'custom_value',
//   'datetime_format' => 'Y_m_d',
//   'datetime_timezone' => DateTimeZone instance,
//   'csv_delimiter' => '-',
//   'csv_headers' => ['foo', 'bar'],
// ]