Can I adding Samsung Memory Module DDR4 - 2666 MHz - 260 Pin for iMac?

I'd confirm this by going to the Crucial website and looking up the product. Their website offers to check for compatible RAM. Your screenshot shows compatible with laptops - it does not mention iMacs.

You need to buy RAM specifically made for Macs. Here is the crucial website with Mac compatible RAM:

https://www.crucial.com/compatible-upgrade-for/apple/imac-(retina-5k--27-inch--2020)-imac20-1#memory

Welcome to Apple Support Community.

No, you should not. For your model, you should only install either Apple Memory Modules or Crucial Memory Modules:

• Apple Memory Module - Apple
• Crucial

Jack

iMacs are extremely picky about RAM, the only 2 trusted vendors that provide very high quality RAM, that have reasonable pricing, have a lifetime warranty and great service are www.crucial.com and Macsales.com. It is your machine, you can add whatever RAM you want however the only RAM we know will work is either Crucial.com or Macsales.com RAM.

The two recommended brands of RAM have already been posted. Here is a direct link to one of them:

https://eshop.macsales.com/shop/memory/imac-2019-27-inch

Do note that Apple does not manufacture RAM. Instead they use several suppliers including Hynix and Samsung. Also note that many retailers market these same RAM products at considerable savings when compared to Apple prices.