Surly Long Haul Trucker set up for commuting and loaded touring, old Sekine road frame converted to fixed-gear, various beaters and weird bikes, waiting on the frame for my Surly Big Dummy build
What exactly are you trying to do? I can only guess it might be some kind of above-seat-steering recumbent, which always end up needing the handlebars up in the air a ways to clear the rider's knees.
I have had luck when working on a tallbike or two with literally extending the handlebar stem with a length of pipe. I sawed the (cheapo chromed steel) stem off where it would usually enter the headset and welded on the pipe as straight as I could. The other end of the pipe was cut off at a length that would drop it into the lower frame's steerer the proper amount. I hacksawed this end of the pipe at a steep angle to replicate the wedge mechanism of a normal stem. A nut was welded into the end of the wedge piece and 5/16" threaded rod was run up through this nut and out the existing hole in the top of the modified stem, where another nut was threaded onto it to allow the wedge to be drawn tight. I sliced a discarded fork's steerer tube off and machined the inside out so it would slide over the outside of the pipe and used this to steady the top of the extension with the upper head tube's bearing race. I gan take a picture of this if I'm not doing so hot a job of painting a picture...
This system works very well for me but is not without it's headaches. For starters, the size of pipe that is closest to the .75" inside diameter of the steerer tube is about .8" in outside diameter. I had to work on the end of the pipe with sandpaper tape for quite a while to machine it down to a size that would slide (stubbornly) into the steerer tube. You could probably scare up some 3/4" OD steel tubing and avoid this problem all together, or you could weld the wedge end of the chopped stem onto the pipe and use that for the critical fit area.
There are also apparently extentions for steerer tubes, but they only go a few inches at a time. I guess they are designed for raising handlebars a bit more when you are already at the limit of the stem's adjustability. See link: