You cannot convert .sis to .jar. Both are entirely different concept.
*.sis files are made for Symbian OS platform. *.jar is a Java module for a Java-supported phone.
It is not like converting *.avi into *.mp3. You can convert that because they are multimedia format. You cannot convert *.avi into *.pdf or *.doc. The same concept applies to conversion of .jar into .sis.
So stop searching around. And if you really need such software, go and buy a Nokia smartphone, which will support both .sis an .jar files. But remember, you cannot convert *.jar into *.sis.