Repackaging the .jar file

I need to add some jars from JRE7 library to my Android project. But for example rt.jar is in conflict with android.jar from Adroid 2.2 SDK, so I get this error:

Ill-advised or mistaken usage of a core class (java.* or javax.*)
 when not building a core library.
 
 This is often due to inadvertently including a core library file
 in your application's project, when using an IDE (such as
 Eclipse). If you are sure you're not intentionally defining a
 core class, then this is the most likely explanation of what's
 going on.
 
 However, you might actually be trying to define a class in a core
 namespace, the source of which you may have taken, for example,
 from a non-Android virtual machine project. This will most
 assuredly not work. At a minimum, it jeopardizes the
 compatibility of your app with future versions of the platform.
 It is also often of questionable legality.
 
 If you really intend to build a core library -- which is only
 appropriate as part of creating a full virtual machine
 distribution, as opposed to compiling an application -- then use
 the "--core-library" option to suppress this error message.
 
 If you go ahead and use "--core-library" but are in fact
 building an application, then be forewarned that your application
 will still fail to build or run, at some point. Please be
 prepared for angry customers who find, for example, that your
 application ceases to function once they upgrade their operating
 system. You will be to blame for this problem.
 
 If you are legitimately using some code that happens to be in a
 core package, then the easiest safe alternative you have is to
 repackage that code. That is, move the classes in question into
 your own package namespace. This means that they will never be in
 conflict with core system classes. JarJar is a tool that may help
 you in this endeavor. If you find that you cannot do this, then
 that is an indication that the path you are on will ultimately
 lead to pain, suffering, grief, and lamentation.
 

I know there have been several threads about it and things like JarJar, OneJar or FatJar might be good for me. But I don't know how to make any of them work and documentation doesn't really make it clear (for me). I guess they use Ant commands, but I have always used Eclipse built-in builder and now I have no idea how to use neither Ant nor any of mentioned above.

So my question is: how can I repack this rt.jar so I could compile it in my Android project?

Thank you!

---