Sorry, I forgot about this.
It's actually really easy to do. But doing so stops the !say command working.
What you will need to have is version 0.9.9 installed. You will find that on the wiki.
After you have it installed download this script here and move it to the scripts folder. Go to the scripts settings and enable the script and write some words in the "Message" box.
After that, make the welcome clip you want and export it as mp3 and put it on some webserver that has a direct download link. Dropbox is good for this.
Then use the direct download link as the tts api link and leave the locale box empty.
This should work, because I have used it before.