I would suggest doing a search for RF breakout boards. Pay attention to the info out there about which frequency is appropriate for the part of the world you live in. It looks like here in Texas for example I probably should use the 915 MHZ one, but yours may be different. You will hook the breakout board up to an SPI interface on thr Pi side. Look through the documentation provided for the chip that is on your selected breakout board. There are configuration steps you will have to take care of, and there is the issue of communications between the Pi and the other RF breakout board by your door. Yes you will need two of the RF breakout boards, one at the door and one in the Pi. You will also need something at the door to set up that RF breakout, such as some kind of microprocessor. It is a pretty complex problem, but after you set up the processor at the door, interfaced it to the rf breakout at the door, established the communications protocol between the rf breakout boards and then received the signal and decoded it in the Pi and responded in the pi, you will have learned a whole bunch about programming and elrctronics. It is not a simple task but you will really learn a LOT in the process! Go for it!