Heres how to do it without breaking the plaster and lath. If your home still uses knob and tube wiring the law prohibits connecting.

Ceiling Light Fixture Installation Amp Wiring
Old wiring new light fixture. So we removed the cover and the light fixture. The connector could be crimps push in connectors or twist on connectors. Fix old house wiring problems. If your home has older wiring often in the form of copper wires without differentiated coating then you must carefully note the colors of the wiring in the fixture you just removed. Tips for connecting new light fixtures to old wiring bringing older electrical work up to code. Disconnect the wires and move the old light fixture to a safe place.
The old light fixture was lovely but it didnt work in a star wars room. All that was left was the wiring coming out of an electrical box in the ceiling. Wiring to add new lights from an existing fixture in this diagram two new light fixtures are added to one that already exists. From there new 2 wire cable is run to the second new light box. Wiring the new light into place. New 2 wire cable is run from the existing light fixture box to the first new box.
By the diy experts of the family handyman magazine. Bring old light fixtures wired with knob and tube wiring up to code by installing an electrical box in the plaster wall. Next once the box is ready to be installed follow the manufacturer instructions to. Disconnect the old wires once the light fixture is free you will be able to see the wires that attach to the fixture with some type of wire connector.