Indeed, I haven't written the export for MEG data to SPM12.
The export to FieldTrip and import in SPM may work, but I have no experience with processing MEG data with SPM12, so I can't advise on that.
If you want further help, you can post a message on the SPM mailing list.
What you can export are data in source space, as .gii surface or .nii volumes. I received a lot more requests for this feature, which is now commonly used.
https://neuroimage.usc.edu/brainstorm/ExportSpm8
https://neuroimage.usc.edu/brainstorm/ExportSpm12