How do I access Nifti file information in MNE-Python for a time-frequency analysis?

Hi all,
I am trying to do a time-frequency continuous wave transform on the Nifti files provided by the final pipeline in the Human Connectome Project’s MEG N-back task. I am working in MNE and trying to use their mne.time_frequency.tfr.cwt(). I have loaded the Nifti files into MNE, but am unaware of how to get the Nifti’s into the right format to be used by MNE functions that typically use .fif files. Is there a nibable function that could help with this process? Thank you for any advice.