How much for the sensor/lot? Coupled with the going price for a used 7D today... yep, it's very possible, sell the 7D and save from not buying the sensor and get a used 5DII 8)
Or get a 5Dc, or forget Canon and get a used D700, so many FF options for the cheap theses days~
BUT, I am curious, if it is at all possible to stick an FF sensor into your camera, I am very sure beyond the multitude of hardware limitations, the firmware would need a major tweak though