Find an inexpensive bar stool (online, flea market, yard sale) that will coordinate with your chair legs. Use those bar stool seats and attach to the chair legs by pre-drilling and screwing onto the bar stool. Without seeing the chairs, this is tough to suggest anything.

Photo needed.....but without seeing them. Cut off the legs at the bottom of a seat level. find some old bar stools at a 2nd hand shop and attach the top of the bar stool to the chair top

Yes. You can buy the swivel units on Amazon.

Perhaps these sites will help you:

https://www.hometalk.com/28196644/q-any-ideas-on-how-to-turn-my-dining-chairs-into-barstools

https://www.instructables.com/Turn-a-Plain-Stool-into-a-Swivel-Stool/

Yup. You would need to cut the legs off at the seats. Buy the swivel bases and attach the seats to them

This will help if you have upholstered style kitchen chairs: https://www.youtube.com/watch?v=ceUIynn3ULo

Yes, It is a matter of cutting off the bases and replacing the bottoms with swivels. It might be cheaper and easier to just replace.

Hello. Posting your photograph might be the most helpful information you might provide. Showing the type of stools you have and if they’re metal or wood would be able to give us more information to offer best suggestions or solutions.

You would have to buy the swivel mechanisms which can be very expensive. It could work out cheaper just to buy new ones or second hand swivel ones.

Hi,

If you add casters on to the base maaybe. Do post pictures of the chairs you are stsrting with for better advice.

Definitely seems a doable idea! You’d just need to track down the appropriate parts and put everything together. Make sure your connections are good and tight.

here is more info for your project

https://www.youtube.com/watch?v=CgbmbZbLCUQ

Here is a video https://www.youtube.com/watch?v=PDGgMf7g5DQ

Hi Dturkey: In my opinion, for what it's worth, I think the cost and effort wouldn't be worth the aggravation. But, if you want to give it a go, check this site:

https://www.instructables.com/Turn-a-Plain-Stool-into-a-Swivel-Stool/

Good luck

Here's a post that might help - https://www.youtube.com/watch?v=CgbmbZbLCUQ

You could buy swivel stools, like on FB marketplace, cut the seat off of your chairs, remove the seat from your stools and screw the new seats in their place

Cheaper to buy stools, or buy stools to get the swivel mechanism. Or check this out, they may have the parts you need. Good Luck

https://www.chairpartsonline.com/swivels/

It depends on style of chair. If it s a chair with four legs on each corner, then it may be more difficult. But if the legs are separate from seat I. Stool style you can either buy a lazy Susan apparatus and either remove seat, add lazy Susan and reattach seat to top of lazy susan apparatus. Or you can attach lazy susan to the top of your seat and then add another seat to top of lazy susan apparatus.