Do you mean you NMEA output with corrections for roll mixed in? Or do you just want the F9P’s NMEA stream to also go to RS232 to use with another monitor?
If it’s the latter, that’s not too hard. You get a MAX3232 breakout board, and tee the TX wire to the F9P’s TX for uart1. The MAX3232 has very high impedance on the TX line, as does the Teensy, so it’s okay in this instance to share the line between the two devices (the RX on the Teensy). It won’t pull the voltage down enough to cause issues for either device.