A PPM Encoder will allow you to use any older style RC receiver with only PWM outputs for each channel. Several versions of the 3DR PPM-Sum encoder are shown below:
There are some downsides of using this encoder:
The PPM Encoder does require quite a bit of additional wiring to the receiver.
It uses quite a bit of power making it likely you will need to plug in your battery while doing radio setup with USB cable in Mission Planner.
The encoder also costs as much or more than modern receivers
If you are using this PPM Encoder it is important to know that when you are calibrating your transmitter you may need to hook up your flight battery to the autopilot because the USB port alone can’t supply enough power.