Can I export certificates and private keys from Lets Encrupt provides free certificates to all users via Lets Encrypt.

Unfortunately for security (and process) reasons you cannot export you Lets Encrypt private key out of

In addition to security, a key advantage of Lets Encrypt is that the certificates auto renew. This means that as a user you dont need to worry about certificates expiring or have ongoing processes to manage this.
An implication of this is that Lets Encrypt certificates review every 90 days. This means that even if you could export the certificate bundle/private key, there would be alot of ongoing work required to manually perform the export every 90 days.

You can run self signed certificates in your origin if needed as the browser does not see this certificate. The browser only see’s the valid Lets Encrypt certificate that is active in the platform.