Create sphere on keyframes with expression

Hi,

I've just begun to learn after effects and could use a little help.

I have a small solid wich is moving around on the comp. This is done using expressions.

What I would like to do is first to replace it with a 3d sphere.

And secondly, instead of moving it, create a new sphere on each keyframe.

First I imagined using trapcode particular, setting the particles count to 1. But that is per second, not per keyframe.

Any ideas are appreciated.

/Lars