Can I add Samsung 850 EVO 500 GB M.2 3.5-Inch SSD?

First you should review this IFIXIT guide: iMac Intel 21.5" EMC 2805 SSD Replacement. As you can see you need to take the logic board fully out to gain access the blade SSD socket. This is a very big job! I also must warn you taking the display off requires the proper tools and care as you can easily damage the display just in the process of taking it off. Note the 23 steps needed to just take the display off!

Now the nitty gritty here:

Please review the custom Apple SSD made by SanDisk: SanDisk SSD now compare it to the Samsung M.2 SSD: Samsung 850 EVO SSD. As you can see the connections are very different. These drives are not compatible. You need to use the custom Apple blade SSD and you need to use the correct unit for your system.

As to the memory review this image: Logic board. Just to the left orange marked screws is 4 chips (8 GB 1600 MHz LPDDR3 SDRAM) as you can see these are soldered to the logic board and are not replaceable.

So the bottom line here is this system is not really upgradable! I would recommend you look at getting another system that meets your needs as is, or get a system that is upgradable. If you can swing it I would go with a Mac Pro or if you want a lower cost solution get an older 21.5" system (pre thin series).