Can you use wall and ceiling texture as plaster slab for drying and wedging clay?
The texture powder is very different from plaster.  Plaster changes its chemical form all the way through to become a solid mass of material.  It hardens while it's wet and then dries out.
There are probably differences in formulation for different texture powders, but a common commercial formulation is mainly finely crushed limestone that makes up the bulk of the material.  It has a small amount of a clay that gels to give it a workable consistency that will hold a shape, and a small amount of starch as a binder.
So the mixture is basically mineral powder that's held together only by a small amount of binder that is soluble in water.  Both the starch and clay will lose their ability to hold stuff together when they get wet.  It's stable enough to take latex paint without falling apart, but it won't hold up to being wet for any length of time.  Also, once it gets wet and starts to fall apart, drying it won't give it back whatever strength it had.
It hardens initially by drying out.  It's designed to be a thin layer.  If you were to create a thick slab of it, it would take a very long time to dry all the way through, and wouldn't have any strength until it's fully dry.  If you wanted to make a slab of it, the practical way would be to build it up in thin layers, letting each layer dry.
That said, if the alternative is discarding it, you might as well experiment.  You could try to use it to reduce the amount of plaster.  Trying to mix more than a small amount of texture mix into the plaster would degrade the plaster.  But you could create a thin slab of it (in layers), and then put a thinner than normal slab of plaster on top.  The plaster might keep the texture layers from getting too wet.  The plaster might not bond well to the texture slab, though, so they could separate with hard use.
Plaster is pretty inexpensive stuff.  You are likely to waste more money in other materials experimenting than the cost of the plaster that the texture powder would replace.  If the texture powder packages are still in new condition, and discarding them would bother you, a suggestion: some big hardware chains will let you &quot;return&quot; it for a store credit long after the normal return period, and without a receipt, if they can restock it (it means they order a little less from their supplier, and you will buy something else in the store).