- Is it possible to send keys to a computer connecting the RPi A+ via USB on the computer?
- Will the RPi have enough power?
- If yes, how?
Based on what I inferred(guessed) from your question, no. Plugging in the Pi to a computer via USB using the micro-USB port will not provide you the chance to send data over it since, from what I've guessed, you're trying to use the Pi's micro-USB port. That port is strictly for power only except on the Pi Zero, one is for data and one is for power. Nevertheless, all Pis can only work as a host (you cannot plug it into a USB port and use it as a device or "slave").
You'll need adapters like a USB to serial adapter and some programming skills to pull it off. So, yes.