I use a box mix 
and add a box of pudding and an extra egg. Would this be enough for a 9x13 or would it make a really thin cake? FWIW, what I'm going to attempt is cutting it in half so I have 2 9x6.5 inch pieces and stacking them on the narrow edge to make a purse cake. (then carve to give a little more shape) Wish me luck! 
I think you would need an extender. You need around 7 cups of batter.
I also use box mixes - when I do the 9x13 size I use a box and a half of ckae mix for each layer...
I always use 2 boxes for this size because it makes a fuller cake.
I use one cake mix if it's a Better Crocker. Duncan Hines I use two. However I'm not always happy with the height of my cakes but I find it hard to have the center cooked without the edges over cooking.
OK, I'm not talking serious carving, just rounding the edges a bit. I would use a round, but I'm not sure I like the totally round look.
#2, I would like to use the mix with the pudding for the flavor. I'm making yellow cake with butterscotch pudding...really good! What else would I need to add for an extender? (I already add an extra egg.)
I have had success carving DH cakes (see my purse cake I just posted), so I guess different brands carve differently. One mix will make for a thin cake, like you mentioned, so go with 1 1/2 or 2. I would think stacking the cake narrow end up would not give you much support, and may not look too hot when it's cut into. I would suggest shorter stacked pieces, w/ bc in between, then carved into the shape you want. Maybe I understood you wrong? Just trying to help,
OK, as for the "stacking", this is what I meant. I found one purse cake in the gallery that said they took 2 8 inch squares and stuck them together so they were standing tall instead of flat. I was thinking of doing that, except with a 9x13. So, the finished product would be 6.5x9x4 inches thick. Would that pose a problem?
Here's a CC purse cake article:
http://www.cakecentral.com/article64-How-To-Make-a-Purse-Cake.html
HTH
You should be fine. When I read it at first it sounded like you were going to stack them vertically! I guess I haven't had enough Dr. Pepper yet this morning! 
The purse I did for this past Saturday had to feed 40, so I did two 10" squares, cut in half and stacked - so 4 layers high. It was pretty much a life-size purse, ha ha.
OK, I'm not talking serious carving, just rounding the edges a bit. I would use a round, but I'm not sure I like the totally round look.
#2, I would like to use the mix with the pudding for the flavor. I'm making yellow cake with butterscotch pudding...really good! What else would I need to add for an extender? (I already add an extra egg.)
To answer your question you would add 1cup flour, 1 cup sugar, 1t salt, 1t flavoring (vanilla, butter, almond or whatever) in addition to what you use. This would probably give you really close to the 7 cups of batter you need to make a nice full cake. Also, stick a greased flower in the center to help it cooke evenly and not have crusty edges. 
i use 1.5 box mixes..or 1 with the extender
When I use that pan...I always use 3 boxes..Mine is 9x13x3.
I use one cake mix if it's a Better Crocker. Duncan Hines I use two. However I'm not always happy with the height of my cakes but I find it hard to have the center cooked without the edges over cooking.
do you use a flower nail or a heating core? that should help the centers cook evenly.
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