I have the Porsche DLC copied from another XBox (Only the content).
Is it possible to add it to my (offline, or online if possible) Account so I can use it, since it’s not possible to buy them anymore.
It’s only possible with a Jtag or RGH modified system.
The only DLC files that can be added and played that way are DRM free versions, and there are not many, and definitely not for one you’re asking about. Sorry.
Yes. License transfer would work. The license is currently set to the Xbox that it was originally downloaded on, which is why adding it to your profile/xbox with Horizon will not work on a retail Xbox 360.