You must use OpenSSL and keytool.
OpenSSL for CER & PVK file > P12
openssl pkcs12 -export -name servercert -in selfsignedcert.crt -inkey serverprivatekey.key -out myp12keystore.p12
Keytool for p12 > JKS
keytool -importkeystore -destkeystore mykeystore.jks -srckeystore myp12keystore.p12 -srcstoretype pkcs12 -alias servercert