move particles to specific position

Hi everybody,

to do a "quasi-2D" simulation set the forces in y-dimension to zero (fix setforce all setforce NULL 0 NULL) and use a very narrow box in the y-dimension where I insert my particles.
Now, all the particles have "nearly" the same y-position, but I'd like them to have EXACTLY the same y-position.
Is there a possibility to move the particles manually so that every particles has the y-position =0? How else could I handle this?

Thanks in advance,
kind regards

Sebastian