Yes you can. We find Mac OS's built in text to speech convertors to be one of the best out there. They are designed to help read speech to blind users so to create an audio file for upload to Yay.com you need to put a small amount of effort in. It only takes a few minutes and the steps below make it really easy.
1) Launch "Automator". This is an application that comes with Mac OS and you can find it in your "Applications" Folder.
2) We are going to create a contextual menu that converts any text you select anywhere on your Mac to an MP3 audio file. A menu you can run by "Ctrl" clicking or right clicking on any selected text. So start by selecting the services icon.
3) Next we need to build the menu option (service). Your's will need to look like the below. Just add the modules from the left to the right until it does. You can select any "System Voice" you like. We like Kate, Alex, Oliver, Daniel, Samantha, Victoria, Vicki and Karen. They are the one's that sound the least computer generated to the team at Yay.com!
4) Now go to the File menu and select save to add this service to your contextual menu. Launch "Textpad" or any other application you can type in after that. Type some text you wish to convert or select some random text from a browser if you wish. Once you've typed or found your text, select it and right/Ctrl click to run the service; as we show you in the step below
5) The service works by creating a playlist in iTunes and using the power of iTunes to create an MP3 file you can upload to Yay.com at Dashboard > Config > Audio. You can now see your file in iTunes so just right click to find where your Mac has put the actual source file.
6) Hey presto, there you have it, a computer generated voice MP3 file of your text!