That's what the keyframe feature is for. You can set the FX to be any particular value at any point throughout the clip.
[edit] OR... by "region" do you mean "area" of the video frame? Sounds like you are trying to have one aspect of the video frame to remain in color and the rest to be in B&W... in which case what you need to get into would be the Secondary Color Corrector and/or possibly the cookie cutter and/or bezier mask.
Please describe a little more about what you are trying to do.