OP:
You can "de-fuse" the fusion drive if you wish.
No one's going to arrest you for doing so.
Actually, the iMac will probably run faster if you do that.
And it will KEEP RUNNING FAST because the de-fused SSD will always run at its highest speed.
HOWEVER:
You really should do this ONLY if you have the 2tb or 3tb fusion drive.
The SSD portion of the 1tb fusion drive is very small (32gb) and probably wouldn't work well as a "standalone boot drive".
Once the drives are de-fused, you can install APFS on BOTH of them.
HOWEVER:
I see no great shakes to having APFS right now.
It seems to be causing problems for some folks and outright havoc for others.
I have no intention of using APFS at any time in the near future.
I'll migrate to it only when no other course of action is possible.
Some more thoughts:
BEFORE you do ANYTHING above, you need to do this:
1. Get an external drive large enough to hold your existing installation
2. Get either CarbonCopyCloner or SuperDuper (both are FREE to download and use for 30 days)
3. Create a BOOTABLE CLONED BACKUP of your current fusion drive.
This gives you "an easy way back" if something goes wrong.
If you don't do this, you can still "get back", but it's NOT going to be easy.
AFTER you do the above, do this:
1. Get a USB flash drive 16gb or larger
2. Get ONE of the following: "Boot Buddy" or "DiskMaker X" or "Install Disk Creator"
3. Use one of the above (free) apps to create a bootable USB flash drive with the High Sierra installer on it.
4. Boot from the flash drive and do the install that way.
Much better chance of success if you take this route.