You have to Download the .Jar Apps to your PC.
Now, Visit netmite.com , You will see lots of Upload Forms.
In the Option Two Form, Use the Upload Jar to Browse your .jar File on your PC, Leave every other thing as it is.
Then Click on the Get Apk Button below the Forms, You Apps should be Proccessed and Available with the .APK Format.
Download and Transfer to your Android Device and Start Using.
Now, I know you have not stated what you're trying to do but you can't just convert a jar or tar file to an apk (an apk is an installation file for Android) hoping it will run or install. First off, it has to be compiled for Android. Android's java library doesn't have all the swing stuff that's in normal java, so the app would have to be completely re-written to work on android and it has not been. (This information from this website)
The only 'offline' version I know of is not free (upontek.com - j2me to android)