This is unproductive. Your connection is secured by HTTPS, which is a secure connection. As long as you don’t have any 3rd party root certificates installed, you should be fine without that.
Bitwarden only supports their own encrypted JSON format.
@waldalla Please note that when you import a plain-text .csv file, the Bitwarden Web Vault app will fully encrypt the data locally (using AES-256) on your device before transmitting the encrypted data to the cloud servers (and, as noted by @CJMAXiK, there is further TLS encryption performed by the HTTPS protocol used to communicate with the server). Thus, there is really no benefit in starting with an encrypted file when doing an import.