You essentially need to push AC signals onto the DC power line, and separate them out again. This is common in homes with TV antennas - the power amplifier is put near the antenna, and DC power is pushed up the antenna wire while TV signals come down the antenna wire.
You don't give enough details for an example circuit, but here are the basics:
The power source should have an inductor in series going to the output to block high frequency signals feeding into the power supply and possibly causing regulation problems.
Each unit's power input should be similarly protected with an inductor to filter out the AC signals. Feeding that into a diode and capacitor will make sure the AC signals don't jeopardize your module power.
Prior to the inductor, you'll also attach a capacitor. It'll probably be a low value so that most of the AC signals on the line pass the capcitor, but none of the DC will.
The output of this capacitor MIGHT be usable directly into a microcontroller (with diode clamping) if you have the skills to implement the necessary software to read the now-mutilated data off the line. Likewise you can send pulses directly into the capacitor with the I/O pin.
Check out what that looks like on a scope - the square wave going into the capacitor will look like a decaying spike on the power line. When it comes out another capacitor on the network it will be further changed - just a spike on the line.
Reading these spikes can be difficult, and filtering out noise can be difficult, so if you're running long lines, have a noisy power supply, or running lines near other noise sources then you'll have to implement significant signal processing. Usually this takes the form of AM (ASK - Amplitude shift keying) or FM (FSK - Frequency shift keying) on the line, with data slicers, comparators, tone generators and detectors, etc. Or the equivalent processing in software.
It may seem like a lot of work, but start off with a simple pulse detector on the receiving end and send square waves when transmitting. Use an oscilloscope to understand what's happening, and if you find you need a more complex solution ask again about ASK or FSK detection.
A pulse detector might be a simple software interrupt on change input pin, or a 555 set up as a pulse stretcher.