Apple engineering executive Craig Federighi stated in September 2017 that it would be coming to a future version of High Sierra, but we've only had one update and APFS for Fusion drives was not included in 10.13.1.
The best thing to do would be to follow High Sierra update release notes as Apple makes High Sierra updates available to the public.
Question: "I was wondering is the New APFS (Apple File System) supported yet for Macs with Fusion Drives?"
But... don't worry about it.
If you want to try High Sierra, just go ahead and install it.
HS will install in HFS+ mode on Macs with fusion drives, and the fact that it's not APFS won't make any noticeable difference to the user (YOU!) at all.
BEFORE you do, I suggest that you download CarbonCopyCloner (free to download and use for 30 days), and create a BOOTABLE CLONED BACKUP of your current OS on an external drive.
If you do this, and have problems with High Sierra, the bootable cloned backup makes it very easy to "get back, get back, get back to where you once belonged".
If you DO NOT make a cloned backup, you'll still be able to "get back", but it's NOT going to be easy.
I suggest that you DO NOT "install from the download".
That is -- your chances of a good install will be better if you create a bootable USB flashdrive version of the installer, then boot and install that way.
You need a USB flashdrive 16gb in size (8gb might do if that's all you have), and the FREE app called "Boot Buddy".
Initialize the flashdrive to HFS+ with journaling enable and use Boot Buddy to create the installer.