应该使用try with resources自动关闭资源。
将文件中的行以及相应的用户名存储到 List<String> 读取文件时。读取完成后，将此列表的内容写入文件。
演示：

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
class User {
    private String username;
    public User(String username) {
        this.username = username;
    }
    public String getUsername() {
        return username;
    }
}
public class Main {
    public static void main(String[] args) {
        // Test
        List<User> users = List.of(new User("Will.Smith3"), new User("Ragnar.Lothbrok74"),
                new User("Anakin.Skywalker30"), new User("Cristiano.Ronaldo32"), new User("Lionel.Messi2"));
        try {
            addUsers(5, users);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    static void addUsers(int maxLines, List<User> users) throws IOException {
        // List to store lines from the file plus corresponding username
        List<String> list = new ArrayList<>();
        try (BufferedReader reader = new BufferedReader(new FileReader(new File("Users.txt")))) {
            String currentLine;
            int line = 0;
            while ((currentLine = reader.readLine()) != null && line < users.size()) {
                list.add(line, currentLine + " > " + users.get(line).getUsername() + System.lineSeparator());
                line++;
            }
        }
        // Write the content of the list into the file
        try (BufferedWriter writer = new BufferedWriter(new FileWriter(new File("Users.txt")))) {
            for (String s : list) {
                writer.write(s);
            }
        }
    }
}