はい、その人のフルネームと訪問回数を含む挨拶文字列を返すメソッドをインターフェイスに追加することができます。以下は `greet` メソッドを追加したインターフェイスです：

```typescript
interface Person {
  firstName: string;
  secondName: string;
  age: number;
  visits: number;
  email: string;
  greet(): string;
}
```

そしてこれが `greet` メソッドの実装です：

```typescript
const person: Person = {
  firstName: "John",
  secondName: "Doe",
  age: 26,
  visits: 893,
  email: "JohnDoe@gmail.com",
  greet() {
    return `Hello ${this.firstName} ${this.secondName}, you have visited us ${this.visits} times!`;
  }
};

console.log(person.greet());
```

これはコンソールに次のように出力する：

```
Hello John Doe, you have visited us 893 times!
```

greet` メソッドを使用すると、`Person` オブジェクトのプロパティに基づいて、必要なときにいつでも挨拶文字列を生成することができる。
Yes, you can add a method to the interface that returns a greeting string with the person's full name and their number of visits. Here is an updated interface with the method `greet`:

```typescript
interface Person {
  firstName: string;
  secondName: string;
  age: number;
  visits: number;
  email: string;
  greet(): string;
}
```

And here is an implementation of the `greet` method:

```typescript
const person: Person = {
  firstName: "John",
  secondName: "Doe",
  age: 26,
  visits: 893,
  email: "JohnDoe@gmail.com",
  greet() {
    return `Hello ${this.firstName} ${this.secondName}, you have visited us ${this.visits} times!`;
  }
};

console.log(person.greet());
```

This will output the following to the console: 

```
Hello John Doe, you have visited us 893 times!
``` 

You can use the `greet` method to generate the greeting string whenever you need it, based on the properties of the `Person` object.