Will a 256GB PCIe 2.0x4 from a late 2013 Mac Pro work on a iMac 2017?

Not sure if thats a win here

Keep in mind your iMac supports a newer SSD PCIe/NVMe Vs what you are older Mac Pro had when it was shipped was a PCIe/AHCI. Here’s a visual breakdown of the different interfaces:

The Orange path is what your Mac Pro is currently using and the Purple path is what your iMac is using.

Now it does turn out the Mac Pro also supports PCIe/NVMe running High Sierra! So you do have an upgrade path for it! But here we are using the older drive to replace the newer one.

I hate to say this but I would recommend you just bite the bullet and get a new Apple blade SSD to replace your current wimpy 32 GB drive. But lets look at this a bit more… You are planning on using the SSD as a dual drive config not a Fusion Drive setup correct? Using a 256 GB drive as part of a fusion drive would be a waste! I could see someone going to a 64 GB or 128 GB but even that I think misses the best part of a dual drive setup. If you can swing it I would get a 512 GB SSPOLARIS or a 256 GB SSPOLARIS SSD for your iMac or even your Mac Pro! Here’s a good writeup of the different interfaces Apple has used in their systems The Ultimate Guide to Apple’s Proprietary SSDs

Also before you do any changes to your iMac you’ll need to break your Fusion Drive set, here’s how: How to split up a Fusion Drive