Our company has to enlarge existing openings in a home with hard plaster walls, very brittle. How do we cut this stuff so as to not chip or crack it. We have been advised to cut it, as with a sharp knife, not sawing in or out.

This may not be the best way, but I've used a cold chisel and ball peen hammer. Careful not to do too much at once. It's a good idea to score the surface prior to chiseling.

Jim, do the walls contain wood lath or are they using gypsum for their main substrate? If the substrate is wood lath i would recommend that cutting with a saw preferably a power saw to be your best option. Of course this will make alot of dust and will require good dust masks as well as closing these rooms off from the rest of the house to contain the dust. When you try doing any kind of repair with wood lath as a substrate,the plaster key is generally very brittle due to age and material make-up of the plaster basecoat.Also the lath boards generally run in four foot lengths so hitting them with a hammer and chisel will requir patience and will still ultimetly damage the plaster finish beyond your required opening.

Jim: Usually the best way is to tape off the area to avoid cracking. This will help hold the plaster and you can draw a straight line that's easy to follow. Use the appropriate saw depending on what type of lathe you have discovered is there. Be really careful pulling the tape that is on the raw edge (the edge you cut) Pull from the wall toward the raw edge. You'll see what I mean.
Dan

You might want to use a 4" grinder with a diamond blade. I would use a dust mask and have someone there with a shop-vac right in front of the blade while your cutting.

The Plaster surface is a marble dust/plaster mix, 1/4 inch thick, over sheetrock.

Sounds like maybe you have lucked out and will be able to score with a drywall knife and finish the cut with a board saw used for cutting drywall.