The deal was initially agreed between the two clubs back in May, as Napoli look to make €10 million (£8.5 million) in player sales to get the deal over the line.
Zeballos turned down an offer from CSKA Moscow for a five-year-deal as he only wants Napoli, per transfer expert Fabrizio Romano.
The 24-year-old winger, comfortable on either flank, has scored once while registering three assists in 2026/27 in Argentina's Premier Division.
He's represented Argentina at youth level, too, making 16 appearances for the U17s and scoring three times.
Napoli's new boss Massimiliano Allegri is looking add Zeballos to his squad for next season, having already brought in Rasmus Hojlund, Allison Santos and Vanja Milinkovic-Savic in the summer transfer window so far.