I don't think there's a simple solution to this. You could set manual exposure and actually change the exposure during the pan to compensate, but, even once you've mastered the amount of control required, it will look "odd".
What you really need to do is to reduce the light coming in through the window - either blank it out with a green screen and chroma key in some external shots afterwards or, if the external is vague enough you might be able to put some sort of gause over the outside of the window.
Another possiblility might be to cut in the shot of the window. What I'm getting at is pan towards the window until the shot begins to white out, then cut to a close up looking out of the window (maybe itself panned) then back to another shot panning from just beyond the window to further into the room.
It's important that the shot out of the window is significantly closer than the pans either side as the change in perspective will override the change in exposure in peoples perception (ie the brain will primarily notice the change of view whereas if the focal distance is similar the brain will more likely notice the change in exposure)

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Tim