Adding 2nd external monitor to iMac

Apple released two 21.5" iMacs in 2017. The iMac (21.5-inch, 2017) was a price point model that had only a dual-core CPU, and no Retina display. You said you had a quad-core CPU, which implies that you have the other one.

iMac (Retina 4K, 21.5-inch, 2017) - Technical Specifications - Apple Support

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Simultaneously supports full native resolution on the built-in display at millions of colors (21.5-inch) or 1 billion colors (21.5-inch 4K) and:

• • One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colors, or
  • Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or
  • Two 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors

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Macs do not support driving two displays off a single port unless

• That port is a Thunderbolt port
• The device directly attached to it is a Thunderbolt device such as a Thunderbolt hub or dock.

So

• You could get a Thunderbolt dock, hub, or dual-display adapter that supports two non-Thunderbolt monitors and attach both monitors to it. If you go this route, I would suggest looking at offerings from Other World Computing, SonnetTech, and CalDigit.
• You could get a plain USB-C dock, attach one monitor to it, and attach the other monitor to the iMac. This would be less expensive, but wouldn't work for you if your external SSD was a Thunderbolt 3 SSD (as opposed to being a USB one).

No, that will not work. Please carefully review the Tech Specs for a 2017 21.5” iMac. It will say how many external display that year will support, which is 2. That means it will not support more than 2. If you have a desire to support up to 3 external displays you will need something a Mac Pro or another device.

You also may want to reach out to macsales.com (call them on a telephone) and ask if they sell a device that would do what you want, if they do not then what you want is not possible. The reason I say this is macsales.com specializes in Macs and are a known industry leader in Mac peripherals.