Let’s encrypt places certificates in the following directory:
/etc/letsencrypt/live/YOUR-DOMAIN-NAME/
The first thing you’ll have to do is convert your certificate to .der format
openssl x509 -outform der -in cert.pem -out cert.der
The to import the certificate run this command:
keytool -import -alias your-alias -keystore cacerts -file cert.der
If you would like to import your certificate into the keystore with .pem format:
keytool -import -alias *alias* -keystore cacerts -file *cert.pem*