Keyframing in Color is limited to one keyframe per grade per room. You also of course need a parameter to animate. Depending on which Bleach Bypass tree you're using, you could animate one of it's node parameters, but not the entire tree, because there is no parameter on the output node to animate, it's either active or bypassed.
If you want the entire effect to fade in over time, then apply one grade to your clip without it and another grade with. Render out both grades and then simply do a dissolve in FCP.
hth,
Clay