FCC-approval and "modular" integration
Assuming (1) that the USB dongle itself is approved as a computer peripheral and (2) the dongle has not been modified, then I believe the dongle is considered a separate and independent product (non-module) and does not need an additional approval to be sold with the Pi.  

The safest best is to include the dongle in its original packaging and let the user plug it into the Pi. The of course means that you need to make the Pi's USB port user accessible. 

If instead you want to include the dongle already plugged in and the assemblage is contained in a non-accessible enclosure, then I think you need to do verification testing on the resulting assembly and label the outside to indicate that the approved transmitter is located inside. This makes sense since a person inspecting the full device would otherwise have no way of knowing what certification that are relying on - or even that there was a certified device was inside.