If you have access to Python, the easiest thing would be to create a Python script such as the one below, and run that. It will find all .npz files in the current directory and convert that to .mat.
from scipy.io import savemat
import numpy as np
npzFiles = glob.glob("*.npz")
for f in npzFiles:
fm = os.path.splitext(f)+'.mat'
d = np.load(f)
print('generated ', fm, 'from', f)