Q: Cannot ease in/ease out when keyframing

Zoom out your video window to 20% ore less, than you see the two end points from your animation. Right klick on the endpoints. Thats all.

AFAIK you are right that there is no "smooth" option for most keyframe parameters.

You may have to either:

a)  add a few more keyframes and manually adjust the parameter in these intermediate keyframes so that the parameter has a more nuanced variation from start to finish (but still piecewise linear between each pair of keyframes)

or

b) make a copy of the effect in Motion and modify it - perhaps adding  some parameter behavior - so that it allows for a smoother variation

(it is beyond my competence to achieve that effect, but I am pretty sure that someone knowledgeable in Motion should do it relatively easily)

Just right klick the endpoint and chose between lineair and smooth.

This works for keyframing *Position* - you can choose between Linear and Smooth. I know that, and so does the OP, and he said as much.

The OP's issue is that this possibility does not exist for keyframing most other parameters - in his case it was the offset parameter in the Letterbox effect. 

The same can be said for just about any parameter: you can add keyframes, but you cannot alter how FCP X interpolates between those keyframes; in particular you can't choose Linear vs Smooth for them.