The generated java code you can open in eclipse... But you cannot put back the changes you make in java-code to your app.
The java-code is generated each time you compile your project! B4A does not have a possibility to import-javacode back to the B4A-project either.
Remember; the java-code which is generated is NOT based on java-code. It is based on B4A-Code
Compile your b4a project and then look in the project's Objects directory.
Here you'll find the java and related files that b4a has generated from your b4a source code.
It's the contents of the Objects folder that you want to create an Eclipse android project from.
So copy the contents of the Objects folder to a new directory and try to import it into Eclipse as an existing android project.