Can't modify the Dissolves that are in Final Cut Pro 10 in motion.
There is no "Open a copy in Motion" command for transitions, the way there is for titles and effects, but that does not mean you can't do it.
The default cross dissolve is actually a very simple Motion transition, which can be redone in under one minute.
If you create a new transition in Motion, you'll see that it is just a straight cut:
The standard cross dissolve is obtained thus:
1) Extend "Transition A" all the way to the right, and "Transition B" all the way to the left, so that they last for the whole duration of the transition.
2) Open the Inspector for "Transition A", and in Properties, Blending add a keyframe for Opacity at the beggining, set it to 100%, go to the end, set it to 0%
3) Do the dual thing for "Transition B", so that opacity goes from 0% to 100%.
You can save this and check in FCP X that it works exactly as the built-in cross dissolve.
Now you can take this as a starting point and modify it to suit your needs.