Dynamic switching of radio: Change between wireless protocols in the 2.4GHz
If you go low-level enough, anything is possible. Your best bet would be to stick with an old WiFi standard -- like 802.11b -- which is comparatively simple to implement. But when I say "comparatively simple," keep in mind we're still talking direct-sequence spread-spectrum and a rather complicated coding scheme (I think it's called complementary-code keying). You'll need to find a transceiver that supports that technology along with the simpler GFSK modulation and frequency-hopping that 802.15.4 uses. That might be more difficult than you think. Then you'll have to write a WiFi stack and an 802.15.4 stack. Sounds terrifying... good luck!