upgrading my mid 2017 iMac, 256 blade ssd rot a 1 TB blade ssd

I think you got things confused, your system can support newer macOSs. The issue is two fold, the newer OSs use a different file system (APFS) Vs what you currently have (HFS+). So the systems firmware first needs to be updated so a drive which holds your OS can boot up under it. But there is a second issue here as Apple also setup a unified driver for storage. So now you need to newer driver for the OWC PCIe/NVMe SSD drive within the newer firmware and OS.

So to upgrade your storage you need to upgrade your OS your system is running. Then in that process both the systems firmware an the OS on drive will be upgraded.

So that solves the firmware issue, but we still haven’t installed the OWC drive so how can I get the OS installed after putting the drive in?

The easiest way is to create a bootable OS installer using a USB thumb drive. Using the OS installer you can prep the USB drive and then after swapping out the drives you can then boot-up under the external drive which you have copied the OS installer to running it directly from the USB drive

Well, the alters things a bit. Depending on the management software you may not be able to overcome it. The rub here is some alter the firmware.

So you need to create an external bootable drive. Grab the OS installer from here How to download macOS and follow this guide to setup the external drive Create a bootable installer for macOS

Then using this external Thumb drive boot up under it to see if you can reformat and install the OS on what you have now. If that works then upgrading to a larger SSD is possible.