If you're talking rock salt like the kind you would use on roads or in ice cream makers, you should not put that in a worm bin at all - it could kill the worms, so there is no need to substitute it with diatomaceous earth. OTOH, diatomaceous earth should cause no issues in the worm bin.

Hopefully you are meaning rock dust as opposed to rock salt as DE would be no comparison to rock salt.

You don't need to add anything like rock dust to a worm bin. There will be plenty of minerals provided by the diversity of food scraps.

I never use or buy rock dust except when it was sent to me with the worm factory. Powdered eggs shells is as far as I go in that direction.