Aluminum steering tubes can be threaded.

Back when aluminum forks were new, and threadless headsets and carbon forks didn't exist yet, aluminum forks were offered with the Buyer's Choice of steel or aluminum steering tubes. If you bought a fork with a steering tube that was too long, you could use spacers or cut the steering tube down. If the threading did not extend down far enough, the steering tube a threading die could be run down the create threads.

When I bought a Specialized Kinesis aluminum fork for my Schwinn 754, I had a choice of steel or aluminum steering tubes. I picked a steel steering tube, and had it cut down to fit my 21" frame. I don't remember the exact weight numbers, but the aluminum steering tube on the same design fork (I thought that with the aluminum steering tube the complete fork was incredible light) was about $50 more at the time. It would have saved me another considerable amount of weight (another 8-10ozs, if I remember correctly) over the aluminum fork with steel steering tube (the aluminum fork was already over 8-10ozs lighter than the original steel unicrown fork).

Why not just get a 1" threadless headset for the bike and use the carbon fork as it is?