Running an alternator and SMPS in parallel
With a synchronous SMPS, you must add your own "OR-ing diodes" from the SMPS output to the power bus.
It doesn't matter that most SMPS have diodes on the input.
Most synchronous SMPS, when it sees its output voltage is "too high", will pull current from the output and store it on the "input" capacitor.
If a person pulls the output voltage too high for too long, that "input" capacitor will eventually fail from over-voltage.

If a power supply -- such as practically all alternators -- have diodes on the output, then those diodes can do double-duty as the "OR-ing diodes".

With "isolation diodes", sometimes called "OR-ing diodes" (either the internal built-in ones of an alternator, or the external ones you'll probably need to add to a switching regulator, or both), you can connect two power supplies to a system and it will work just as you expect -- whichever power supply is turned on and generating the higher voltage will supply almost all the power to the load.

You may want smoothing capacitors at the load.
When the SMPS is turned off, no current flows from the power bus
"backwards" through the output OR-ing diodes into the smoothing capacitors of the SMPS, so those capacitors are effectively isolated and do nothing to smooth the voltage produced by the alternator.

"Can power supplies be connected in parallel for redundant operation?"

"Considerations for parallel power supply outputs"

"Using Isolation Diodes for Parallel-Redundant Operation"

"Connecting Switching power supplies in parallel is possible if certain precautions are observed."