I wanted to try a raspberry WASC a few months ago, so I emailed Rebecca Sutterby (the creator of the WASC recipe ... her website is www.sugarcreations.com
... her cakes are absolutely A-M-A-Z-I-N-G!!!!!) and asked her what changes I should make to the recipe. Here's what she told me:
Use about 10 oz. of frozen raspberries per 2 boxes of cake mix. Thaw the berries in a strainer over a large bowl, and save all of the juice. Substitute all of the juice for some of the water (put the juice in a liquid measuring cup, and then just add water to get to the 2 2/3 cup mark). Mix in the berries right before you pour the batter in the pans.
The cake was absolutely fantastic! I put raspberry filling between the layers and iced the cake with cream cheese icing. Oh my gosh, it was to die for!!!!
I'd say that you can use the same recipe variation for any kind of berry (strawberries, blueberries, cherries --- I know, not a berry, but ...
---) ... the possibilities are endless!!! Hope this helps!