Z-coordinate: setting to zero

Hi!
I have to work on a 2D file exported from another sw.
If I look at the drawing I can see that almost all
the objects doesn't belong to a plane, but they have
a non null Z-coordinate.
So I ask if anyone knows how can I convert all the
non null z-coordinate to the zero value.
I know that I could select some objects and then
setting the Z = 0 in the properties window...
however not all objects permit to do this..
so If I select all, I can't do it in one shot ...
and make it one by one means wasting a lot of time...
Thanks.