Animate on Path of Merged Shape

Effects --> Generate --> Vegas. otherwise the limitations Rick laid out apply. That said, for this kind of work it's usually better to just draw stuff in Illustrator and create a compount outline there, then import everything into AE to add effects. Less trouble, more control.

Mylenium

Merge overlaps shapes and paths. It does not create new ones. Your only option is to use Layer/Auto Trace, then choose Current Frame, the Alpha Channel, reduce the Tolerance, increase the Threshold, reduce Minimum Area, and Corner Roundness. That will give you a mask on a new layer that you can copy and paste to any position property to create a motion path. You will have to fiddle with the settings to reduce the number of vector points and still accurately generate an accurate motion path. If you zero out everything, you'll have a keyframe on every frame of the motion path. Turning on Preview will give you a pretty good representation of the mask that will be generated.

Something like this will give you a good starting point.