I think this should work....
1) Connect the mobile to the PC using the data cable
2) Select the File transfer mode on the mobile [You'll get a new drive in My Computer]
3) Copy the file on to the Phone memory....[You can create a special folder to keep JARs eg. abc]
4) Disconnect the phone and browse to the folder you created using File Manager [In Organizer in K320i] and install the JAR file.....