Typegraphql resolver for existing class

Question

How can I write a resolver in typegraphql for a class I can not edit myself?

Normally I would have the situation that I write a class e.g.

@ObjectType()
export class User {
  @Field()
  f_name!: string;

  @Field()
  l_name!: string;

  @Field()
  email!: string;
}

And then I can add a resolver which returns this class

  @Query(() => User)
  user():Promise<User>{
    some code which returns a User
  }

Now I am using a package which defines a class transactionOutput and I would like to use this object as a return for my graphql query. How can I accomplish this without editing that class?