Here is a trick:
Set up your scene with the animation you wish (e.g. animated clouds, some animated geometry - whatever you wish).
Do not enable motion blur.
Render your image as a sequence with pass limit set to 1 (it could be more if you wish to get better overall quality) and save each frame in CXR format.
Then open CIE, use the "merge" option in CIE, and pick your CXR frames.
You should get something similar to motion blur as the frames will be simply stacked on each other.
You can also use a different format and stack them in other software.
To get a different motion blur curve, you would probably have to adjust the shape of the animation curves in the 3ds Max curve editor.