Basically, no.
You could theoretically use a piece of DVR-type hardware like the eyeTV to pass a signal through to the iMac, but the resolution would be lower than the screen's native and there's usually enough lag with a system like that to make gaming awkward--even a half-second can badly screw up most games (where it makes no difference with TV).
Plus it's ~$100 of hardware; you can get a 20" monitor for not much more than that.