SO I just bought an external microphone but just noticed that it uses a pretty rare battery, an RC2 3V one and I was wondering; Is there any chance I can somehow power it up using a USB cable?
USB ports will produce 5 volts, which is more than the 3V battery produces. Here's what Wikipedia says about USB output power:
The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices. A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0. 5 volt half an amp.
This means that to use the microphone with USB power, you'd need to have a voltage regulator that drops the voltage from 5v to 3v.The NJU7211L30-ND regulator will probably do the trick.
You could also just buy the battery at a site like this one for around the price you'd pay for a voltage regulator. It's entirely up to you.
Sure you can.... If the battery is a 3V type you may need to drop 5V USB supply to around that. This can be done easily by connecting three ordinary silicon diodes in series so that their total voltage drop is about 2volts.
Use these 3 series connected diodes between the USB +5V and the positive terminal point of your microphone. This will ensure the microphone gets the 3V. You may need to bypass this 3V to the common ground by a 100uF capacitor.