How can I change (add) the code to the .dex file on Android?

Question

Java

Good day!) I am writing a code compiler program for android. With the javassist library, I can add or change the function code of my dex file like this:

final ClassPool pool = ClassPool.getDefault();
pool.appendClassPath(new LoaderClassPath(getClass().getClassLoader()));
pool.appendClassPath("./android.jar");
final CtClass compiledClass = pool.get(targetClass);
final CtMethod method = compiledClass.getDeclaredMethod(targetMethod);
method.insertBefore("{ java.lang.System#out.println(\"doA() is called.\");}");
compiledClass.writeFile(targetFolder);

But for the functionality of exporting and importing my projects, I need to do this with dex files that will be saved on the sd card for example.
So here's the question. How can I add code to a .dex file that is separate from the application?