Hhm, someone may have a better solution but you could try this adapter from RobotShop:
http://www.robotshop.com/en/hex-hub-...5in-2wide.html
3/8 in. diameter comes to about 9.5 mm. If you have something able to press it on tightly you may be able to get the shaft in. Or if you can turn the size of the shaft down slightly, you'll be set. If you manage that you may need to shave down the key as well but you'll have to see.
Not ideal, but I think it is doable.
EDIT:
I think these are actually 3/4 in. Hex, not 1/2 Hex. See the section "Specification" lower down on the linked page.