Stencil is like an overlay that masks out particles and parts of particles. You can achieve the same thing using masks in other applications.
A better solution might be to use and area emitter mask.
1) add an area emitter to the stage
2) Make it the same size as the stage
3) Add a background image, sequence, or movie that contains alpha
4) open properties dialog for stage emitter
5) check “mask area emitter with image(s)
6) click the mask “options” button, etc.
The area emitter mask is different in that it defines where the particles are created. It does not cut off parts of particles, or restrict where particles can go after they are created.
Alan.
http://www.wondertouch.com