Isn't that just the 'Chat to contact' option on the right-hand side of the person's contact?
It's the blue circle with the speech bubble. If you click on that you get a standard chat window and the option to tick a box for SMS (if you want it).
To make it all really usable you need to add mobile contact numbers to your contacts list, and also set up your skype options to return SMS messages to your own mobile phone, as otherwise the person you text can not reply to you so easily.