That's exactly it. I was able to successfully introduce an N/A intake, but it only works with the original game-provided bus, not the expansion pack busses. Would be fun on twisty, hilly roads - especially with the smaller busses and manual transmission to really increase the workload and reward good driving efforts (perhaps equally punishing bad driving as well). The same way it's more rewarding driving low powered econo boxes quickly than super cars...
Yeah it's pretty annoying how nothing can be added to the expansion. I'm sure with enough time, someone will find a solution though