There's a feature in FontLab that allows you to turn a corner into "Looped corner":
I'm looking for a way to do that in Illustrator. What I do now, is to add an anchor point besides an existing corner. Then, I extend both anchor points in the path direction to create a loop. This method doesn't perform well when there are handles in either side of anchor.
Another method I use is to break the anchor using Scissors, then switch to Extend Path tool (Part of VectorScribe) and extend both sides and join them:
I think there must be a better and shorter way for doing that like a script or plugin.