Trying to build a bike, confused about fork/headsets/compatability I have a fork with a 1" steerer tube, and a frame with a 1.5" head tube. 

The steerer tube is slightly short, and when inserted into the frame, is flush with the top of the head tube. The fork is threaded. 

My question: Is it possible to extend and adapt this fork to threadless, and what kind of headset (reducing, normal?) would I need? (the frame I bought said the headset size was 1 1/8 traditional press-in) 

Or is all this just stupid, and I should just buy a new fork with a 1 1/8 steerer?
I would get a fork with a steerer that fits the frame. The expense and risk of modifying your existing fork won't be worth it. There were some mountain bike forks with replacement steerer tubes but they didn't allow for changes of size or type of steerer. They were 1 1/8" threadless and so was the replacement. You also want to match the fork rake/angle that the frame maker designed into the frame.