Well, the real answer is a little bit different than yes. Unless you are the manager of the U21 team in question you cannot just put your player on that team. You can send a message to the coach of the country in question and ask him to consider your player for the team. It is up to the U21 NT coach to decide. In your message to him, it is a good idea to include your player's skills and maybe the training regimen you have him on.
Once I scored a basket that still makes me laugh.