Unfortunately the Movie Maker team did not provide a customization API (application programming interface) to allow developers to create additional transitions or effects.
They did provide this in the prior version of Movie Maker (Windows Movie Maker 6.0) and there is growing list of available transitions and effects for that version.
I can understand your hesitation in using another version of Movie Maker, but the version that came with Vista can also be used on Windows 7 or Windows 8. And it will be installed in a separate folder, so you can use it or the latest version, as the needs of your project dictate.
Downloads:
Custom Transitions, Effects and Titles
Note: all those have been used by numerous members of the community throughout the years, but if you still are concerned, then I would skip it. In that case, the answer is "no" there are no custom transitions available for the latest version of Movie Maker.