So after a long leave of absence, I come to find that there is a B1.00. I go to my HiB key page, get the build, and blow some shtuff up. Same old.
Then I find out it's on steam now. I jump for joy, and realize that it has LAN capabilities. I nearly lose my socks in excitement. Then I realize I'd have to buy it... Again. And own two copies of Cortex Command. (Which wouldn't be bad, mind you, but I'd have no use for one of them.)

Is there anything I can do, besides paying for a second copy, to get my HiB copy transferred to Steam?

On the HiB page, press "Retrieve Steam Key".

Edit: Local multiplayer is split-screen, not LAN.