Summary

Replace the factory flanged Acme nut with this and get 10 mm more build height without new linear rods or up to 50mm with new rods.

Update:
You can now "tinker" with a simplified version of this on TinkerCAD!
https://tinkercad.com/things/7EuLauiVIcG

Instructions

Print on the Simple 2014 with 0.2 layer height and 30% rectilinear infill. Then replace the stock part. Be careful when you pull the y carriage off. I lost one ball bearing out of my linear bearing. After it's replaced home your z axis and then issue M211 Z125 (or 115+Lenght you choose) followed by M500 to save it. I also had to change my printer settings in Repetier Host.

The original part was drawn in a trial copy of Geomagic Design. The STL files uploaded here are from it.

Update 20150115:
I've redrawn a simplified version of this with FreeCAD and TinkerCAD. I used FreeCAD to generate the ACME threads and imported them into TinkerCAD. I've not had a chance to print this yet, but you can now Tinker with it!

https://tinkercad.com/things/7EuLauiVIcG

Printrbot Simple 2014 Z-Ext