How to access java-classes in the default-package?

In fact, you can.

Using reflections API you can access any class so far. At least I was able to :)

Class fooClass = Class.forName("FooBar");
Method fooMethod = fooClass.getMethod("fooMethod", String.class);

String fooReturned = (String)fooMethod.invoke(fooClass.newInstance(), "I did it");

Use jarjar to repackage the jar file with the following rule:

rule * <target package name>.@1

All classes in the default package of the source jar file will move to the target package, thus are able to access.

You can’t use classes in the default package from a named package.
(Technically you can, as shown in Sharique Abdullah's answer through reflection API, but classes from the unnamed namespace are not in scope in an import declaration)

Prior to J2SE 1.4 you could import classes from the default package using a syntax like this:

import Unfinished;

That's no longer allowed. So to access a default package class from within a packaged class requires moving the default package class into a package of its own.

If you have access to the source generated by groovy, some post-processing is needed to move the file into a dedicated package and add this "package" directive at its beginning.

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Update 2014: bug 6975015, for JDK7 and JDK8, describe an even stricter prohibition against import from unnamed package.

The TypeName must be the canonical name of a class type, interface type, enum type, or annotation type.
The type must be either a member of a named package, or a member of a type whose outermost lexically enclosing type is a member of a named package, or a compile-time error occurs.

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Andreas points out in the comments:

"why is [the default package] there in the first place? design error?"

No, it's deliberate.
JLS 7.4.2. Unnamed Packages says: "Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications or when just beginning development".