Probably you want to import a “fake” SSL cert in JRE’s trustcacerts for avoiding not-a-valid-certificate issues. Isn’t it?
As Jon said, you can do the job with keytool:
keytool
-import
-alias <provide_an_alias>
-file <certificate_file>
-keystore <your_path_to_jre>/lib/security/cacerts
Use “changeit” as the default password when asked (thanks Brian Clozel). Ensure to use this runtime at your server or launch configuration.