The RDF dumps are marked as BETA because we are still in a phase where
we need to apply breaking changes in quick iterations. We can not do
this any more if they are not beta any more.
Even if it is technically possible to import a .json dump and turn it
into an RDF tripples dump, this is not what we do. We are using the
dumpRdf.php maintenance script to create the RDF dump, which is using
the code I pointed you to.
Maybe instead of asking if a specific solution exists, you can start
with explaining your problem, what you have, and what you would like
to achieve? I believe this opens more possibilities for people to
answer and help you.
Best
Thiemo