Thank you to all who tried to help. Here is an update.

1. My monopod, it turns out, does not have a common bolt thread on the point. Rather, it has, from Sirui, a proprietary kit that includes an adapter and a sturdy looking tripod that can double as a tabletop tripod. One of you showed pictures of the same thing but on a different model. So I if I want to add a 3 footed adapter I have to spend about twice as much money. The large, flat, circular foot looked interesting, too, but it would not work on my gear. That's a shame.

2. All who point out that a monopod with feet is not a tripod and should not be used that way are correct. Obvious, but true. I have a tripod - two or three, actually - but carrying them around and getting them in and out of the car and then quickly setting up is not what I want right now.

3. When shooting birds, even in good sunshine, I use F8 for DOF and, besides, that is all my reflex lens offers. For the obvious reasons, I use at least 1/000 shutter speed. That means that I'm at ISO 200 already. I am thus already in the corner of the envelope. I'm simply not as steady as I was when young. I use trees, railing, car windows, etc. when available. But the monopod could offer another method. Another stop of shutter speed would help but can introduce noise in the image. I've experimented with this and found that sharpness is reduced from 1/000 to 1/2000 for that reason.

4. My 500 mm reflex lens offers AF and is very sharp. But it doesn't have any image stabilization of its own. My camera, a Sony 𝜶6500 has in-body stabilization but that's much less effective than when the lens also has it. When I rented a Tamron 150-500 the net effect was greater stability. But I haven't made up my mind to spend $1400 quite yet.

4. When shooting with a monopod some axes of motion (such as vertical) are stopped or steadied but some aren't. For example, yaw (left-right motion) is not affected. With a foot such as a little tripod device and a friction-set ball, more of the possible motions are either eliminated or slowed. Pitch is another that can be better. Some of you agree and some of you don't. I lean toward increased resistance to camera motion. The physics and geometry are easy. Real life is sometimes more complicated. At an equivalent of 750 mm, even the tiniest motions are detrimental to sharpness.

Second update:

The big and expensive option from Siri is attractive but I decided to try a minimalist approach. The suggestion of a large rubber foot gave me the idea that something that would give me a little bit of resistance to movement of the monopod might help. I did some internet searching and eventually found something with a larger bottom surface than the monopod and with a hole into which my monopod's existing "foot" or hoove would fit. I had to order two to get them to accept the order and because Amazon didn't carry it I had to pay for shipping. Here is what I ordered. I will probably post another update at some point. BTW, the Perry video was helpful.

The foot you see here was the only one I could find that would fit and still offer some bottom diameter. I would have liked a bigger bottom surface but so far have not found that.

When jumping in and out of the car, carrying the monopod, etc., this will be easier and lighter than the $75 Sirui product. Of course, a really good tabletop tripod as a bonus might still be the way to go.

I have a SIRUI P-326 monopod and I'm quite happy with it. However, I'm thinking about adding a 3-feet attachment to the bottom end. Amazon has several at reasonable prices.

Does anyone have actual experience with:
A. adding feet to existing monopod?
B. a particular brand or model that was either notably good or notably bad?
C. experience with a monopod that has feet, even if it came with them, to suggest that this is a good or bad idea?
D. any "gotchas" to consider that are not immediately obvious?

No, I don't want to use a tripod.

Thanks in advance.