Minecraft 360 DLC Unpacker
Open the DLC file in LeFluffie and extract the file inside it.
Now unzip the Minecraft DLC Unpacker. Open Quick BMS Editor GUI. Click Script File, Open, and select minecraft pck unpacker.BMS. Now go to Archive File, Open Archive, and select the Skins1.pck file you extracted with LeFluffie. Now go down and click Extract and save where you want it extracted.
Go to the folder where it was extracted and you'll find a ton of png files with the different skins. Go download your favorite Minecraft skin png files, rename them with the file names in the folder and over write.
Now download Minecraft Importer and extract it. Open Reimport, select the same BMS file you used earlier. Now it will ask you to select your archive file, which will be the skins1.pck file you extracted earlier, and then after that select the folder where you extracted your skins.
Then just import the skins1.pck file back into your DLC file with LeFluffie and try it out.