As far as I know, there isn't a direct way to change the feedrate in corners. You could create some intelligence in the post to get your desired result though. Depending on the how your post is outputting arcs (IJK, R, or point to point) you may be able to make a condition where if the corner being turned has a radius or IJK value less than a certain amount, reduce the feedrate. Change the feedrate parameter value to 'Expression'. I don't think it would be possible if you are outputting corners/arcs point to point.
