Short: Power over this kind of cabling is something that's done quite commonly in different ways.
First: You might want to think twice about using Wifi für stuff if you have a perfectly usable LAN nearby. Using LAN doesn't congest the radio spectrum (which you might want to use for devices that can't use LAN). LAN is much less prone to snooping or interference than Wifi (restricting physical access to LAN Ports is easier than securing Wifi AND restricting physical access to the components). There are cameras with LAN connection that have PoE built in (more on that later)
But despite that, let's just talk about Powering your devices over ethernet cabling.
Cutting up an USB Cable and splicing in Modular (RJ45) plugs
This is the simplest solution and really cheap
Pro:
Contra:
- Prone to error
- Possible stability issues
- Has to be done by hand
- Might not work if your cables are long and/or your device needs more than just a few mA of current
Network cabling has a relatively small diameter. Thus it has a "high" resistance which results in a voltage drop along the wire that's proportional to the current it carries and it's lengt. So if you have a 5V Power Supply at one end there will be less than 5V at the other end. Also, if the load current changes, the voltage might drop (or rise) even more at the load end (induction in the wire). Also, if you miswire something or plug in a device other than your self built "adapter", stuff might break.
Injecting more than 5V, regulator to 5V at receiving end
Pro:
- can be diy
- relatively cheap
- voltage stability good, receiving device protected
- can work over greater distances and with higher current
Con:
- might not be worth it (PoE injectors and splitters are cheaply available)
- adheres to no official standard
Example: you take a 12V power supply and put +12V on four wires and GND on the 4 other wires. Than you use a voltage regulator (preferably a switching regulator) that has protection on the primary side and buffering on the secondary side plus a few resistors to make the device believe it's getting power from a "real" USB power supply and connect the USB device that needs to be powered.
Use PoE injectors / splitters
Pro:
- can adhere to a known standard (interoperability)
- can be safe
- can be extendable
Con:
PoE injectors and splitters are relatively cheap. Make sure you buy devices that adhere to a standard (and don't just do what was proposed in the diy solutions above)
Use a PoE switch and IP-Cameras that Support PoE
Pro:
- fully standardized
- you get support if anything doesn't work
- stable LAN connection for your cameras
- No additional hardware required that flaps around somewhere
- Hardware of different manufacturers can be mixed to a great extent
Con:
- Probably the most expensive solution