You can use a threadless quill adapter like this:


The one pictured above is for a 1-1/8" steerer (I assume you have the newer 1-1/8" forks).

Or a stem extender like this:


You can buy the BBB BHP-20/21 Stem Extender above from Wiggle for $17.68.

Adjusting bearing preload will be a bit tricky though.
1.) Attach the threadless stem tightly onto the adapter.
2.) Insert the assembly into the fork's steerer, including spacers.
3.) Make sure everything is snug and rattle-free as you can make it.
4.) Gently turn the tightenting bolt little by little while pressing down on the stem to load the bearings. The tightening bolt works like a compression bung.

Do the adjustment with the bike on the ground and the front wheel attached.

Good luck.