We're using the Humble Widget which is a bit different than being in the actual Humble Bundle - there might be something to upload keys, but I'm not sure I've seen it yet and their publisher tool is pretty simple.
EDIT: Just checked my account and there is definitely no way to add Steam keys for our Humble Widget. I may be able to add them through our Humble rep, though. I'll have to contact him to see.
That's weird. I would definitely check with your rep though, as I've seen other studios offer Steam keys through their Humble widget, like Runestorm who added 2 Steam Keys (Shadow Warrior Cleanup Detail and Santa's Revenge) to their current DRM-free offering, and they still plan on updating it with a steam key for Viscera Cleanup Detail, when it launches on Early Access soon.
Good luck with your launch! I'll be looking forward to playing on Steam.