The answer is ‘Yes”, you can put a spinning reel on a fly rod from a technical perspective.
The reel seat of the fly rod will accommodate the feet of a small spinning reel allowing you to attach it.
But the question is why you would want to do it?
For starters, you can’t put an opaque and comparatively thick fly line on a spinning reel, which means you are constrained to use only a monofilament line.
That means there is no “weight” in the line to allow you to cast and you are pretty much restricted to really just dropping the fly on the water or flicking it a short distance.
Being only able to target fish within a rod length or so of you will really compromise your success in fly fishing.
What Is the Right Reel for a Fly Rod?
Fly fishing rods need a dedicated fly fishing reel. Fly reels are quite simple reels with no gearing. The reel has a spool that rotates around an axle that sits at 90 degrees to the rod, like a baitcaster. But it is much more simple – the handle is attached directly to the spool of the fly reel and when you wind it, the spool turns and recoups line.
Fly reels have a drag too, allowing you to apply resistance to stop a fish taking line too easily.
Are There Any Circumstances Where a Spinning Reel on Fly Rod Makes Sense?
So, as you can see there really are no circumstances where it is advantageous to put a spinning reel on a fly rod.
The only situation I could see it making sense is if an ultralight spinning reel is the only reel you have! If money is particularly short, I suppose you could swap it between the fly rod and the spinning rod.
But again, you aren’t likely to catch anything fishing at such close range anyway.