Here is how I imported the company's digital certificate (key pair) from iSeries machine:
1. get the KDB file from the iSeries file structure and move it to a windows box
2. Use a java-based tool named iKeyMan.bat to open the key db file. The tool is shipped with Websphere product.
3. export the key pair to an existing "jks" format file which is java keystore. The file will server as the keystore, and can be used to sign jar files.