1. First of all export the real Certificate + key pair

1.1. From IIS as example: Edit bindings

1.2 Thеn: Edit -> View -> Copy to File 

1.3 Export the private key

Notice: if you are going to export already present SSL certificate from Windows store used by IIS, you may face the problem that "Export" entry is not clickable because "Mark this key as exportable" was not checked by importing that key. In such case you must have Аdministrator rights and use free tool called "Mimikatz" to export such keys from Windows keystore.

1.4 PKCS #12 (.PFX) Include all certificates + Export all extended properties

1.5 Set the pass to "secret" and save as file like cert.pfx

Here you may continue as described in https://support.tsplus.net/solution/articles/44000038469-how-to-convert-different-ssl-https-certificate-formats-to-java-jks-

A: Converting SSL certificate from *.pfx/*.p12 format to cert.jks