How to import a .cer certificate into a java keystore?

If you want to authenticate you need the private key – there is no other option.
A certificate is a public key with extra properties (like company name, country,…) that is signed by some Certificate authority that guarantees that the attached properties are true.
.CER files are certificates and don’t have the private key. The private key is provided with a .PFX keystore file normally.
If you really authenticate is because you already had imported the private key.

You normally can import .CER certificates without any problems with

keytool -importcert -file certificate.cer -keystore keystore.jks -alias "Alias"