I think you would have to hack the firmware to do so, if it's possible at all. To help protect their installed bases of legacy 802.11b/802.11g products, Apple, Cisco and Intel do not support 40MHz-wide channels in the 2.4GHz band because that would mean occupying 2 out of the 3 non-overlapping channels available there. Still, you could switch to a Linksys adapter... even though owned by Cisco, Linksys products support wide channels in the 2.4GHz band. Or you could switch to 5GHz. There are over a dozen non-overlapping channels up there in the U-NII bands.
