I'm only coming with adaptors that are advertised for Ahead systems (I'd have to read more on those to see how they differe from standard threadless headsets). A couple of examples:

Satori Heads-Up Stem Extender
http://www.sjscycles.co.uk/zoom-head...rks-prod18251/

Zoom HEADSUP2 Steerer Extender
http://www.jensonusa.com/Zoom-HEADSUP2-Steerer-Extender

BBB BHP-22 Steerer Tube Extender
http://www.wiggle.com/bbb-bhp-22-steerer-tube-extender/

I haven't found any comprehensive reviews on them, mostly on whether they are safe/recommended/reliable/etc.