Convert .cer certificate to .jks
keytool comes with the JDK installation (in the bin folder):
keytool -importcert -file "your.cer" -keystore your.jks -alias "<anything>"This will create a new keystore and add just your certificate to it.
So, you can't convert a certificate to a keystore: you add a certificate to a keystore.
Just to be sure that this is really the "conversion" you need, please note that jks files are keystores, a file format used to store more than one certificate and allows you to retrieve them programmatically using the Java security API, it's not a one-to-one conversion between equivalent formats.
So, if you just want to import that certificate in a new ad-hoc keystore you can do it with Keystore Explorer, a graphical tool. You'll be able to modify the keystore and the certificates contained therein like you would have done with the java terminal utilities like keytool (but in a more accessible way).
Export a certificate from a keystore:
keytool -export -alias mydomain -file mydomain.crt -keystore keystore.jks