Most phone companies provide a way to translate e-mail into SMS messages. Essentially, they set up special e-mail addresses; if you send e-mail to one of these addresses, the company sends an equivalent SMS message to the desired phone number. You can find an international list of such e-mail addresses here.
To send SMS messages to a particular user instead of normal e-mail, specify the appropriate e-mail address in the user's contact record. Since you can only specify one e-mail address per person, you can send either SMS or e-mail, but not both.