Only for circles? Rotate 45 degrees and add extremes, then rotate back if you like.
Or in the middle of all segments of all sorts of shapes? That is a task for a script:
for thisPath in Layer.paths:
# step backwards through nodes so we do not mess up point indexes:
for thisNode in thisPath.nodes[::-1]:
if thisNode.type != OFFCURVE:
# on-curve index + 0.5 is the center of the segment:
pathTime = thisNode.index+0.5
thisPath.insertNodeWithPathTime_(pathTime)
(Sidenote: Please use the term on-curve because anchor means something else in fontmaking land.)