CiviMail: Send to Individual Contacts (not just Groups)?
Not exactly what you want to do, but if you install the Resend Mailing extension, you can select one or more contacts from search results and use the action drop down menu to resend a mailing to those specific contacts.
Alternatively, you could just add the new recipients to the original group you used for the mailing and resend the mailing, subtracting the recipients of the original mailing and thus only sending it to the new group members.