i don't know if this is possible or not, I don't have CS3 yet.
you could try opening 7.0 and simply importing your CS3 project. That's how you normally open one AE project in another.
If that doesn't work, maybe check to make sure you have the most recent version of trapcode. it's a very popular plugin, and I imagine there would be a huge uproar if it didn't work in CS3.
or, you could simply render out your animation from CS3, take the rendered animation into 7.0, add your particles on a new layer, then render ONLY that new particles layer out of 7.0, and take that layer into CS3. Of course, every time you want to make a change, you'll have to do this process over again, but it works.
