You can add the smtp to the user. However he will never get a hotmail e-mail directly to that account hosted by you since you are not the owner of “hotmail” domain. You can probably setup a pop3 account and then it will “retrieve” his hotmail to this account and just grant him access to it.
He can currently add a POP3 account for his own hotmail to his outlook without you doing anything at all on his behalf. It really depends on what you are trying to achieve for this user. Aquacer0 is right, you do not own the hotmail domain to add this as an accepted SMTP address to his existing exchange account. What is it exactly you are trying to achieve? Perhaps there is another solution which you need instead.