The basic idea for powerline communication is inject information/data with high frequency carrier into powerline.
You can use transformator or capacitor to inject the signal. Constructing the transformator is depend on your choice of carrier frequency, your powerline impedance and how far your powerline receiver.
Vacuumschmelze GmbH make powerline communication transformers.
National Semiconductor once made IC's for such systems, and I think some of their application notes described transformer construction.
Try searching their on-line files. Many others have also entered this market, so it should be possible to find what you need.