Yes, you can mix plaster without a drill. If you want to mix plaster with a power tool, a cordless or electric screwdriver is best for this. First, remove the batteries so the tool does not get bogged down when mixing water and plaster together. A standard drill will also work but it is more difficult to control the speed of the spinning action which makes it easier to over-mix or under-mix your product. You may need some help holding things together while mixing but these devices are more expensive than an inexpensive device like the one mentioned above and barring that just find something to hold everything in place while you're mixing!