Can I use soapstone dust to make gesso?

​Soapstone dust is not a good substitute for calcium carbonate in paints and grounds. The particle size of the dust produced by carving is probably not optimal for paint and grounds anyway. I would just dispose of it (wearing a particle filter mask), and if you want to use hydrated magnesium silicate/talc in the studio for inks or ceramics, select a cosmetic grade that is finely processed and product free of asbestiform particles.

Matthew is probably right about the particle size issue
The comment below was written by Bruner Barrie who runs Sculpture House
Since there is various type of soapstone or Talc Block that may also be called Serpentine or know by other names it would be difficult to answer this question without more information.  I cannot see why any of the different types might not be used with Gesso although the porosity might be different within the different types. Montana, India African, and Alaskan stones all vary so I would suggest the person take a small amount and mix up test batches and lay around to see what they might like best. That is what I do quite often when trying something new and exciting and it seems to work well in the long run.
Hope this helps.
Thanks and Sincerely,
Bruner
 

You could certainly make tests of your powdered stone and perhaps you could find a use for it. I have doubts about is appropriateness for silverpoint. I generally use ground silica for that purpose. The hardness of silica facilitates mark making. Perhaps yours would be too soft. Who knows unless you try.  On the other hand. Marble dust, ground gypsum, and ground silica are about the cheapest materials used in art making. You have to decide for yourself if it is worth it to you to substitute your ground stone powers for materials of known qualities and a history of successful application.