Yes. Pro Logic IIx, given a 5.1 source, processes the 2 surround channels to create 4 new channels, 1 for each surround speaker. It doesn't modify the fronts or LFE.
Having said that, on the quality front, you may want to check how good the Dolby processing is. Some receivers will only perform Dolby post-processing at 48kHz, so you'd lose quality from a 96kHz source (even from the fronts). And I don't know of any that do post-processing at 192kHz. The more modern a receiver is, the more likely it is do do 96kHz processing. Most don't actively advertise 96kHz processing, apart from Denon, so it can be hard to tell (unless the receiver refuses outright to process 96kHz sources, rather than downsampling).