I'm about to go out but have a lot of people asking me so I'm going to quickly tell you all how to do it.
1. Get the desired DLC off your HDD and put it on your desktop or wherever.
2. Download Lefluffie (Google for it)
3. Open Lefluffie go to file then open file (Choose the DLC you wish to edit)
4. Click on the contents tab and this will have all the files extract what you want to edit
5. Right click on the file you want and extract it then modify it.
6. Right click on the file you want to replace and click replace file (Choose the file you wish to replace with)
7. When you are all done click on the security tab and check sign for Dev Live or sign for CON (I use CON but some people say you need to use Live)
8. Click the fix button
9. You are now done go and replace your DLC with your modified one.