Getting DLC

Hello! I have a Xbox360 Slim that is not modded. I would like to be able to access some old DLC i played back in the day (mostly cod maps). As far as I am aware, if I have the DLC file, I should be able to use horizon to change the account id associated with it and add it to my 360 with a flash drive. Is this the case? i cant seem to find any info on doing stuff like that for vanilla consoles. What file type would I need? Any help would be much appreciated!