i think that the wetting in that game should be visible. therefore it would be good some user could create a mod that changes the wetting then upload it to a mod site like mod the sims.
Having it look like a wetting after the fact is impossible at this point, but what you could do is edit a texture so it looks like pee.
Here's an example of that in Sims 2:
http://www.moreawesomethanyou.com/awesomeland/index.php?view=4661 (http://www.moreawesomethanyou.com/awesomeland/index.php?view=4661)
http://www.moreawesomethanyou.com/awesomeland/index.php?view=4663 (http://www.moreawesomethanyou.com/awesomeland/index.php?view=4663) pee stain
it should be possible in the sims 3 but it wont work in the most recent version because electronic arts prevented the use of custom content.