I have tried this with an addon maps dlc and I have tried it with a single car addon dlc and neither worked. In both cases I have added the addon dlc into: newmods/dlcpacks/720s/dlc.rpf (for example with the vehicle) and the modified dlclist.xml into newmods/common/data. That's exactly how it is described in the wiki in the New Way of Modding section.
In the case of the addon maps, the maps didn't load and in the case of the vehicle it just gave an Invalid Model error when trying to spawn it. I have added the log file from when I was trying to load my addon maps dlc called roadmaps.
Has something in a later update stopped this from working, or is this an ongoing tool/mod that will work in the future?
ClosedIV.log