Hm.
Add to citizen/common-sp/data/dlclist.xml (copied from citizen/common/data/dlclist.xml):
<Item>citizen:/dlc/rdd/</Item>
… and extract the rdd/ folder to FiveM.app/citizen/dlc/.
This presumably will work, assuming you used ArchiveFix to fix the RPFs (both outer and inner), or add OpenIV.asi to plugins. (it’ll be ignored for MP, so should only apply to SP)
EDIT: seems OpenIV.asi doesn’t work, but using ArchiveFix on the inner rdd.rpf and rdd_metadata.rpf (extract, affix, put in) and then the outer dlc.rpf seemingly worked:
