I had thought several times over the years about the possibility of doing this with a kokarde that has the much smaller, screw mounting hole.
Two things always stopped me from altering a kokarde with a small sized hole:
1.) The M91 side post sized hole on the kokarde has a flared or raised edge to the diameter of the circular hole. This could not be accomplished by simply "drilling" a larger hole in the kokarde to accommodate an M91 side post.
2.) I also thought about the possibility of chipping or significantly damaging the baked enamel paint on the kokarde, if a new larger size hole was made in the kokarde. I believe the original kokardes were painted and oven baked after the hole had previously been punched in each individual kokarde.
Best Wishes,
Alan