Hmm...should be possible.
Look here for a good explanation of what you're tying to do only substitute the BT network interface for the hardwire LAN interface on that page.
Basically what you want to do is to bridge the wi-fi connection with the BT connection, linking the two. Both will be on separate subnets of course, but the BT subnet will be routing it's traffic through the wi-fi subnet to get out to the internet.
I'm really no good at teaching or explaining anything.
Good luck!