The first problem is that you're asking two different questions.
First, I'm assuming you'd like the surfer dude to sound like keanu when he was in bill & ted's excellent adventure, and toby to sound like the guys from "dude, where's my car", etc.
This is, in fact, a voice change, and the good news is that yes, you can get other voices for the mac. For example, infox makes a bunch:
http://www.assistiveware.com/ivoxsamples.php?voice=JulieiVoxHQ
Personally, I like the Julie voice, but I don't speak french, so that's no help to me.
However, I haven't run into any of the three voices you are looking for. If you find some, please add links to this thread.
As to your second question, (changing what is spoken from "it is 19 hours" to "Dude, it's 19 hours and the surf's up!"): no, there is no easy way to do that.
If you're familiar with any programming languages, you could easily whip up your own. Ex: if you're in a terminal, the "say" command will say whatever you want, and there is a man page that will explain how to use any of the available voices. If you feel the last line was in greek, then no, there is no way to do what you want. Sorry.