The inverter is necessary to connect to the FrSky SmartPort. That solution and other alternatives are discussed for native APM Planner 2 support here:
The simplest way should be to use a standard inverter (e.g. http://www.banggood.com/FrSky-Receiver-Upgrade-Lite-FUL1-p-930265.html) with the built-in APM 2 functionality and the serial (UART) port of the NAVIO+ (which I presume is mapped in correctly via their APM HAL driver).
But I think the best way, which I have personal experience of, is the Teensy solution. Because standard telemetry mapping is too limited and we have a full Pi computer, we don’t need a Teensy of course and can tweak the program to do more, for example LiPo battery cell monitoring (also described in that link).
At least I will be taking a look at this because I have the same requirement. But for me at least it’s only nice to have, because my radio is only intended to be a backup to WiFi/ground-station (phone/tablet) display.