要修改已存在的方法，需要使用JavacTask提供的框架和API来进行修改。具体来说，可以使用com.sun.source.util.TreePath类和com.sun.source.tree.MethodTree接口来获取该方法的语法树，并通过JavacTask提供的修改器来进行修改。以下是示例代码：

import com.sun.source.tree.*;
import com.sun.source.util.*;

import javax.annotation.processing.*;
import javax.lang.model.*;
import javax.tools.*;
import java.util.*;

@SupportedAnnotationTypes("*")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class MyProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set annotations, RoundEnvironment roundEnv) {
        for (Element e : roundEnv.getElementsAnnotatedWith(annotation)) {
            TreePath path = Trees.instance(processingEnv).getPath(e);
            MethodTree methodTree = (MethodTree) path.getLeaf();
            JCTree.JCMethodDecl jcMethodDecl = (JCTree.JCMethodDecl) methodTree;
            // 修改jcMethodDecl
        }
        return true;
    }

    private void modifyMethod(JCTree.JCMethodDecl methodDecl) {
        ListBuffer statements = new ListBuffer<>();
        // 添加需要的语句
        statements.append(...);
        methodDecl.body.stats = statements.toList();
    }

}