It doesn't appear that there is. The plug-in's certificate database is named CACERTS, and for me (Windows 98) that file is stored in:
c:\program files\javasoft\jre\1.3\lib\security
The certificate database there already has some of the standard CA's stuff in it. For more information search these forum pages for an article written by Abraham Khalil titled:
"Steps to create your own self signed certificate with java plugin working"