Converting nii format to fif

Hi, I was trying to plot a Macaque brain using mne.viz.plot_alignment, what I have is a .x3d file, and a nii.gz file. I notice that in MNE, this function and mne.read_source_spaces using FIF files. Is there any way that I can convert .x3d to .fif? And I have the data like SEEG but from macaque, which means I have MNI coordinates and voltage data, so I want to using stc.plot_3d, which means I need to use mne.read_source_spaces

Hi, I suppose I may be wrong about the file. The x3d is not labeled using ROI. So the data I should use might be .nii.gz format. So my question is, how to convert .nii.gz file to .fif?

Thank you