Hi Chris,
Good question. The answer is yes, and I've just experimented with it last week. You need two modifications:
1) solder an extra 1ohm resistor in parallel with Rsc (the current limiting resistor for 34063 switching regulator), this is to increase the current output of the +5V line. On OpenSprinkler 1.3 and 1.4, you will find a resistor slot parallel to Rsc, and that slot is exactly designed for this purpose.
2) get a USB Male B to Female A converter, such as this one:
and plug it into OpenSprinkler's USB port. Then you can plug your WiFi adapter's USB power cable to the converter and it should work.
I will soon write a post about this. If you can't figure out how to make it work, just wait for the post.