How can I build shower niches into an old plaster and lath wall?
What I did in that same situation was tear the lath/plaster down to the nearest stud and then install drywall.  It is very likely that the thickness of your lath/plaster will be thicker than drywall (1/2").  So what you will need to do is shim below the new drywall to even it out with the old covering.  What I did was use 3/8" plywood as substrate then drywall over it.  Then just regular drywall finish (tape+mud) to make the transition between the plaster and new drywall.