Create your shape using the Paint Stroke Tool, Bezier Tool or B-spline tool. Ad a Write On Behavior. Click on the Shape Tape, go to "Advanced" and check the Dynamics box. It's very cool. Hope this is what you where looking for.
You can have a paint stroke's cells be treated as already-emitted particles, but if he's more interested in defining an emitter's shape, he should use the "Geometry" option in the emitter settings.
To do this:
Draw a shape in the Canvas that represents the size and shape you want your emitter to be.
Create your emitter (or add an emitter preset to your project).
With the emitter selected, go to the Emitter tab in the Inspector and change the Shape popup-menu to "Geometry". Note the Shape Source image well that appears in the Inspector.
Drag the shape you drew in step 1 into the Shape Source image well.
This will tell the emitter to use that shape object as the pattern for emission. You can turn off the shape so that it's not in the way, but you can edit its points to alter the emitter's pattern, if you like.