First, use KeyTool GUI to export the keystore in a Private Key and Certificates (PKCS#12) format.
Second, issue the following command with OpenSSL:
openssl pkcs12 -in file.pfx -out file.pem
You might also want to read this.
TJ on Jan 5, 2005
Like many others I was Gooogling for answers on how to convert a SunJava JKS key-store to another format, in my case the Microsoft PFX format, so the same certificate could be used by Sun's jarsigner.exe and Microsoft's signcode.exe.
I ended up writing a couple of small tools, putting a package together, and writing a comptehensive illustrated guide in the hope it'll save others a considerable amount of time and stress working it out.
“Trusted Code-Signing for Free!”
Here's how to get and use a FREE trusted Thawte digital certificate to sign your Java JAR and Microsoft CAB code archives, to create trusted applets for downloading over the Internet, and to convert the Java JKS key-store to Microsoft PFX Personal Information Exchange format to share the same certificate with Java JAR files and Microsoft CAB files.
http://tjworld.net/software/codesigning/