Which Njoy product you have? If you have Npro, then you can just pop out the liquid cartridge using a paper clip through the little hole in your mouth piece, and drip few drops of e-liquid to the cartrige filler, then re-insert the cartrige into your mouth piece.
The best juice is very subjective - and to be honest, I haven't really read really "horrible" juice reviews in most cases.
Njoy doesn't recommend it because it halts their sales of their overpriced cartriges. If you want, you can even buy those cartriges much cheaper from elsewhere, and chances are, they'd be as good as Njoy ones, if not better.
Npro cartriges are rn4081 cartriges - out of my head, I can just tell you that Totally Wicked and Pure Smoker sells the cartriges. You can get liquid from pretty much any vendors in supplier sections down there.